Compared with phased array radars, Multiple Input Multiple Output(MIMO) radar has lower transmit power density. In low SNR, the advantage of space diversity is not clear and the detection probability decreases. Aiming at this problem, a weak target Tracking Before Detecting(TBD) algorithm based on Successive Target Cancellation-Dynamic Programming(STC-DP) is proposed. Firstly, the algorithm screens the track points by statistics and integrates signals, reducing the complexity and the noise accumulation. Then, detect targets successively based on successive target cancellation. The proposed algorithm has advantages that it can overcome track furcation and detect multitarget effectively. The detection probability and false alarm probability are derived. The relationship between false alarm probability and detection threshold is given. Complexity analysis and simulation show the complexity decreases compared with traditional TBD algorithms；and the SNR is decreased by 5 dB in the same false alarm probability and detection probability compared with traditional detection algorithms, improving the detection performance.