In order to improve the detection capability of orthogonal frequency division multiplexing (OFDM) modulated radar for high speed weak target, a high speed weak target detection algorithm for OFDM modulated radar based on coherent Radon transform was proposed. Firstly, the echo signal model of high speed weak target and the range migration which generated by the long-time coherent integration was analyzed, then the coherent Radon transform algorithm was used to long-time coherent integration along the slant line of the range migration, and the parameter estimation method for the existence of velocity ambiguity was studied. Theoretical analysis and simulation results show that, the proposed algorithm has better detection performance in the low signal-to-noise ratio (SNR) background compared with incoherent Radon transform and moving target detection (MTD), and it can estimate the parameters effectively.