Microinjection with force sensing plays an important role in biomedical domain. However, most of microinjectors with force sensors lack enough load-bearing ability to support a pipette holder, which is needed to inject foreign materials into cells. In this paper, a new force-sensing cell microinjector with novel compliant small-stiffness mechanism is presented. It maintains large load-bearing ability and meanwhile high sensitivity. The small-stiffness is realized by the combination of a positive-stiffness and a negative-stiffness mechanisms, both generated from a constant-force mechanism. The output force of the microinjector is the force difference of these two mechanisms. When the microinjector moves forward, the forces of these two mechanisms are both increased, and hence the difference will be extremely small. The positive and negative-stiffness mechanisms are designed and calibrated. A prototype microinjector is fabricated and tested. Experimental results show that the sensitivity has been improved more than twice that of the previous work, verifying its effectiveness in force-sensing microinjection.