Insect cell is the host of baculovirus in Baculovirus Expression Vector System (BEVS). However, the insect cell counting is an obstacle that constrains the efficiency in BEVS. In this paper, an insect cell counting method based on Transformed Sliding Band Filter (TSBF) was proposed according to insect cell cultivation manner. The proposed method was then applied to insect cell image datasets, and results exhibited that the average relative error rate was 2.21% compared with manual counting. Growth curve evaluation showed that this method was suitable to the protocol of cell cultivation. These exciting results proved that the proposed method was an ideal automatic counting tool for inset cells in BEVS.