The method of overthehorizon detection based on the small aperture array is used to overcome the quite large aperture of the traditional OTHR. Since the performance of signal processing is depressed by using the small aperture array，this paper presents an orthogonal frequency division multiplexing linear frequency modulation(OFDMLFM ) waveform design method. With the aid of the signal model, the range resolution of the coherencesynthetic is analyzed. In order to suppress range sidelobe, the convex optimization is proposed. The simulation shows that OFDMLFM signal can improve the performance of the range resolution. The peak sidelobe level is -26.28 dB and the average of pulse compression is less than -50 dB. According to the result，the capability of noise and interference cancelation are increased, and the target detection performance is improved too.