Abstract: The research of semi-strapdown seeker was carried out, the control principle of semi-strapdown seeker was showed and the integrated stabilization loop of semi-strapdown seeker was proposed. The theory of tracking differentiator was introduced and the simulation was carried out. The simulation study result shows excellent track and filter capability of tracking differentiator. In order to cut down the noise of velocity measurement in semi-strapdown seeker, the tracking differentiator was used. The simulation and hardware in loop result show that the velocity of platform frame is well estimated by tracking differentiator, the stabilization precision of semi-strapdown seeker improves 80% , which testifies the feasibility and validity of the tracking differentiator in semi-strapdown seeker.