- 2017 American Physical Society. We theoretically investigate optical detection of a weak classical force acting on a free particle in a dissipative coupling optomechanical system with a degenerate parametric amplifier (PA). We show that the PA allows one to achieve the force sensitivity far better than the standard quantum limit (SQL) over a broad range of the detection frequencies. The improvement depends on the parametric gain and the driving power. Moreover, we discuss the effects of the mechanical damping and the thermal noise on the force sensitivity. We find that the robustness of the force sensitivity is much better than the SQL against the mechanical damping and the thermal noise is achievable in the presence of the PA with a high parametric gain. For the temperature T=1 K, the improvement in sensitivity is better by a factor of about 7 when the driving power is set at a value corresponding to the SQL with no PA.