Abstract: To investigate the direct radio frequency(RF) sampling techniques for multiple band signals, a new method of finding the minimum sampling frequency for multi-band signals is proposed. Based on the principle of bandpass sampling, a searching algorithm can be derived using the corresponding relationship between image frequency and sampling frequency. To correct the calculation errors due to truncation of float-point number, the algorithm constraints are amended and the implemental flow chart is built. Simulation results show that the proposed method is efficient with relatively low cost and it is convenient to be applied in engineering.