This paper reports a cavity optomechanical signal amplifier driven by optical force in photonic circuit platform, with an amplification factor as high as 4. High amplification resolution is obtained through the precise optical force control. Different with conventional optical amplifier, the proposed design realizes the optical amplification using an optical force actuated nano-structure. The signal amplifier consists of a tunable ring resonator, an actuation ring resonator and a mechanical beam. Utilizing the nano-structure deformation by the optical force, a small input optical signal is amplified to a large output signal. An amplification factor as high as 4 is obtained experimentally and the modulating frequency is up to 4.2 MHz.