Charge overflow phenomenon will arise when CCD is receiving strong signals. With CCD without overflow drains, the parameters of the imaging system have to be set properly to avoid charge overflow. Based on the charge overflow phenomenon when using HAMAMATSU Frame-Transfer CCD to image not so strong targets, according to the paper, it is caused by the long-time charge accumulation of the image section during line readout process. To remove the overflow signal effectively, multiple frame transfers are carried out before integration time. Integrating sphere tests are also performed to testify the reason and the solution.