An abnormal oil-water separation occurred in the terminal FPSO crude oil treatment system of an oilfield in Bohai. Through analysis of the process conditions， it was found that the main reasons were a decrease in process temperature， changes in crude oil composition， and waste oil discharge from the upstream closed drain system. In response to the existing problems， targeted measures were taken， such as high-temperature oil well extraction， mixing of two series of crude oils， and optimization of closed drain pump discharge methods. Ultimately， the crude oil treatment effect was improved， ensuring that the crude oil in the lower hold was qualified.