The South China Sea summer monsoon usually breaks out earlier in the La Nina year, but a late onset (6th pentad of May) appeared in 2021, though 2020/2021 was the year of La Nina. Using NCEP/NCAR reanalysis data, the reasons for the late onset of the South China Sea summer monsoon in 2021 are analyzed from the perspective of tropical sea surface temperature anomaly (SSTA) and intraseasonal oscillation (ISO) northward propagation. The results show that La Nina did weaken the Western Pacific subtropical high in spring, especially before April. However, the tropical Indian Ocean sea surface temperature in the winter and spring continued to be warmer, offsetting the impact of La Nina, especially in May. The influence of La Nina was less than that of the tropical Indian Ocean, resulting in the stronger Western Pacific subtropical high in May and the late onset of the South China Sea summer monsoon. In addition, due to the influence of La Nina, the Western Pacific subtropical high in April was weak and the background barotropic southerly in the South China Sea was weak, which unfavored the equatorial ISO in the South China Sea propagating northward. As the influence of tropical Indian Ocean SSTA became more and more significant, the Western Pacific subtropical high has gradually strengthened, and the background barotropic southerly did not extend to the area south of 10°N until late May, casusing a late northward propagation of the equatorial ISO in the South China Sea in 2021, which is the opposite of the climatic state. This is also an important reason for the late onset of the South China Sea summer monsoon in 2021. The tropical Indian Ocean and Pacific SSTAs together have an impact on the onset of the South China Sea summer monsoon through competition, so it is very important to pay attention to the development of the two in winter and spring.