Abstract: To address the challenges of combustion stability, hydrodynamic safety and wide-load peak-load regulation following the significant increase in boiler capacity and parameters, and to verify the feasibility of the technical solution for the world's first 700 MW ultra-supercritical circulating fluidised bed (CFB) boiler, a systematic account is provided of the entire process of start-up, commissioning and operational optimisation for the 700 MW ultra-supercritical CFB boiler at Yunnan Energy Investment’s Honghe Power Station. The boiler employs an M-shaped single furnace chamber design with a single air distribution plate and no external bed, featuring a furnace cross-sectional area of 575 m². The main steam parameters are 29.3 MPa/605 ℃, with a reheat steam temperature of 623 ℃, and it burns high-moisture lignite. Based on tests of air distribution plate resistance and cold-state fluidisation characteristics, the minimum fluidisation air flow rate was determined, and the ignition procedure was optimised to reduce the time required to raise the bed temperature to 600 ℃ by 2 hours. Optimisation and commissioning were carried out across the combustion, steam-water, control and auxiliary systems: air flow rates were calibrated to ensure a primary air flow deviation of ±3%; the distribution of coal-feeding air and secondary air was adjusted to ensure a bed temperature deviation of ≤30 ℃; the oxygen concentration field was optimised to ensure an oxygen concentration deviation of ±0.3% at high loads; a load-bed pressure setpoint curve was established, and bed pressure fluctuations were maintained at ±0.5 kPa through a combination of continuous and intermittent ash discharge control; a feedforward-feedback composite control strategy was adopted to ensure a water-cooled wall temperature deviation of ≤30 ℃, a high-temperature superheater outlet wall temperature of ≤620 ℃, and a reheat steam temperature fluctuation of ±3 ℃; Adjust the parameters of the fan variable-frequency drives to ensure that current fluctuations in the primary fan are≤8 A and in the secondary fan ≤5 A; employ model predictive control to optimise bed temperature and bed pressure regulation, achieving coordinated control of the SCR/SNCR and combustion systems, resulting in NO_x emission concentration fluctuations of ±5 mg/Nm³, a load change rate of 3 %/min, and a 100% automatic engagement rate for key parameters; optimise soot-blowing strategies and the desulphurisation system to stabilise flue gas temperature at 132−136 ℃ and ensure desulphurisation efficiency of no less than 99.7%. The results show that, following commissioning, the boiler’s thermal efficiency exceeded 93.5% and the coal consumption for power generation was below 282 g/kWh, both of which surpassed the design targets. A 168-hour full-load trial operation was successfully completed in a single attempt, with an average load factor of 99.4%, while SO₂, NO_x and dust emissions met ultra-low emission requirements. The feasibility and advanced nature of the 700 MW ultra-supercritical CFB boiler were thereby validated regarding its external-bed-free design, uniform fluidization in large furnaces, efficient high-altitude lignite combustion, and ultra-low emissions, providing a critical engineering reference for subsequent ultra-supercritical CFB boiler designs.