In order to achieve precise force control of collaborative robot， an in-situ integrated torque sensor is developed by using the deformation of the flexspline of the harmonic reducer. Proposed a residual ripples compensation method based on Fourier series model to improve measurement accuracy. Firstly， the strain law at the bottom of the flexspline was analyzed， and the sensitive grid structure of the strain gauge was designed based on the principle of sinusoidal superposition； Then the Fourier series model between the remain ripples and the angle of the wave generator was established to compensate the residual ripple； Finally， a test platform is developed for loading test， the results show that the output errors of the self-made torque sensor and the standard torque sensor are 0.988% and 0.253% when the instantaneous excitation and step load are applied under the static state of the wave generator； Under the rotating state of the wave generator， three kinds of loads， i.e. step change， sinusoidal change and man-made collision in the process of sinusoidal change， are applied. The output errors between the self-made torque sensor and the standard torque sensor are 3.822%， 4.247% and 4.456%. The compensation method proposed in this paper can effectively reduce the interference of residual ripple on the sensor output， which is conducive to the realization of accurate measurement of joint torque.