The flow cytometry nowadays includes non-imaging-type and imaging-type. The non-imaging flow cytometry has a high throughput, but it cannot provide cell morphological information. The imaging flow cytometry which can obtain cell morphological information has a low throughput due to the frame rate limits of the conventional CCD array imaging sensors. Aiming at this problem, an ultrafast flow cytometry quantitative phase imaging system based on optical coherent detection is proposed and designed in this paper. Its effective line scan imaging speed is up to 100 MHz. The cell imaging experiment results show that the proposed system can acquire high-contrast quantitative cell phase images and a theoretical throughput of 50000 cells per second.