Cell microinjection is extremely crucial in biomedical domain. Currently, cell miciroinjection is mostly operated by manual method and position-based robotic method. However, the success rate and survival rate of the injected cells are relatively low. Force-assisted robotic cell microinjection can provide force feedback and protect the cells from excessive force. In this paper, a new cell microinjector with novel small-stiffness mechanism is proposed. The small-stiffness property is achieved by combining together a constant-force mechanism and small-stiffness mechanism with force intercept. The constant-force mechanism is used to offset the force intercept of the small-stiffness mechanism when the constant-force mechanism works at constant-force range. The constant-force mechanism is modeled, simulated, optimized, fabricated and calibrated. Results show that the designed constant-force mechanism can offset the intercept force well, because the variation of constant-force value is under 0.1 N during more than 1 mm constant-force motion range. The combined low-stiffness mechanism (without force intercept) can serve as an extremely sensitive force sensor for cell microinjection application.