Aiming at the problem of particle filter track before detect (PF-TBD) based on small target, the number of sampling particles increases exponentially with the spatial dimension, and the calculation volume increases sharply. This paper proposes an improved tracking algorithm named serial resampling track before detect (SR-TBD). The proposed algorithm serially samples the position space and velocity space of the particle state to realize the effective accumulation of energy between search frames, and reduce the number of states to increases the amount of calculation, and improves the convergence speed of the algorithm. The comparison between algorithm simulation and traditional algorithm shows that the improved algorithm proposed in this paper has been improved in terms of convergence speed and calculation volume.