A cutting device for a sewing machine includes an upper-blade positioning device and a lower-blade positioning device. The upper-blade positioning device includes an upper blade upper blade fixed at a head portion of the upper-blade holder, and a spring and a restricting member sleeved on a body portion of the upper-blade holder. A restricting pin is engaged between an upper-blade drive member and the restricting member. A guide member includes a guide hole for insertion of a rod portion of the upper-blade drive member and is fixed to the base. The upper-blade drive member is inserted through a guide member of the base and moved by a connecting rod. The lower-blade positioning device includes a lower-blade holder, an adjustment member. A lower-blade shaft has one end fixed to the base and another end extended out of the base, rotating the adjustment member can adjust the position of the lower-blade holder.