The NCEP/NCAR reanalysis data was used to perform a circulation analysis on the 2019 height field, OLR field and wind field and was used to calculate the physical quantities such as pseudo-equivalent temperature, southwest wind and vertical wind shear. Then, the Lanczos filter was used to filter the phase and discuss the relationship between ISO and the 2019 South China Sea summer monsoon onset. Results show that the date of the 2019 South China Sea summer monsoon onset is May 6, and its outbreak is too early. The specific characteristics after May 6 are as follows:200 hPa upper level jet range is expanded and its strength is enhanced; the subtropical high continues to withdraw eastward, and the southwest wind no longer prevails in the South China Sea; southwest wind prevails in the South China Sea area at 850 hPa and convection breaks out in large areas; the characteristic that the potential pseudo-equivalent temperature changes with altitude shows the trend of convection enhancement. In order to discuss the relationship between the 2019 South China Sea summer monsoon onset and ISO, further research reveals that there are 10-25 days of intraseasonal oscillations in 2019. On one hand, ISO is conducive to the early outbreak of 2019. On the other hand, after the outbreak, low-frequency convection from the Bay of Bengal-East Indian Ocean propagated to the northeast many times over time, going through six stages of development-strongest-weakening-suppression-weakest-recovery. It is conducive to the enhancement of the westerly wind in the South China Sea and the maintenance of the outbreak of convective activities, which increases its outbreak intensity.