Ocean Assimilation System (OAS) is an important component for decadal prediction experiment, providing initial conditions. Evaluating the atmosphere response in OAS can provide reference for analyzing results from decadal prediction. We analyzed the interdecadal change in relation between the East Asian Summer Monsoon (EASM) and El Niño/Southern Oscillation (ENSO) in the previous winter based on an OAS on the coupled climate model FGOALS-s2. It shows that two factors impact the performance: ① interdecadal change of Ssea Surface Temperature (SST) pattern in the summer Indo-Pacific Basin related with ENSO in previous winter and ② bias in model response of the western North Pacific anticyclone to tropical SST anomalies. The anticyclone shows steady relation with the warm eastern Indian Ocean. When ENSO’s impact on the summer Indian Ocean is strengthened around the end of 1970s, the OAS can reproduce the strengthened EASM-ENSO relation. However, the trend of intensified EASM-ENSO relation in the OAS is still significant after the mid-1990s due to the stronger link between the anticyclone and the northeastern Indian Ocean, differing with the observation which shows a weakened effect of the Indian Ocean on the anticyclone. In addition, the bias in response to the SST anomalies in the central Pacific also partly contributes to the failure in reproducing the weakening EASM-ENSO relation after the mid-1990s. It implies that prediction skill of interdecadal ENSO impact on the tropical Indo-Pacific SST and response bias of model to SST anomalies may to some extent limit the capability to predict the interdecadal change in the EASM-ENSO relation.