A viscoelastic contact model between surgical instruments and ocular tissue is established on the basis of generalized Maxwell viscoelastic model, and the contact model parameters are identified through force relaxation experiment. On top of that, an admittance control method is developed. The contact force is controlled through adjusting the displacement of surgical instruments. In the proposed method, the traditional admittance controller is taken place by the proportionalintegral (PI) controller. In this way, the ideal differential element in viscoelastic contact model is replaced by the 1st order differential element. After replacement, the amplitude-frequency response at low frequency can be improved and kept stable, especially when the frequency approaches zero. The attenuation of contact force caused by low amplitude-frequency response can be avoided. Some force control experiments on in vitro pig eyes are carried out to prove the effectiveness of the proposed method. The experiment results show that the average error and response time of step force are 4.6% and 2.5 s respectively, no obvious overshot occurs, and the sinusoidal force with certain frequency can be output. The output accuracy of step force meets the requirements of robot-assisted ophthalmic surgery.