The study of P80 on-line solid solution process was tested by Gleeble 3800 thermal simulation testing machine， and the results showed that the open cooling temperature ranged from 870 ℃ to 930 ℃， the lower the temperature， and the higher the hardness， the higher the variance and the lower the hardness fluctuation after the on-line solid solution and aging. Under the premise of ensuring that obtaining single-phase structure and uniform hardness， the process parameters such as open cooling temperature and cooling speed were determined： the open cooling temperature was 860 - 880 ℃， cooling speed was 10 ℃/s， and the re-reddening temperature not greater than 400 ℃. Applying the above process parameters to P80 production practice， the results showed that the on-line solid solution P80 steel plate after 520 ℃ / 3 hours of aging treatment， the microstructure was transformed from bainite and martensitic to granular bainite and lath bainite， hardness was 40.1 HRC， which could meet the technical requirements， and could replace the off-line solid solution process.