To cut down the sidelobe level of radiation pattern, a novel adaptive algorithm is proposed for electronic steering parasitic antenna. The composite objective function in this algorithm takes both directivity and sidelobe level of pattern into account, and the steepest gradient algorithm is selected to search the optimum value of reactive load. Simulations are carried out to validate the algorithm, simulated results show that the levels of sidelobe are both below -4 dB in different beamforming cases, and the front to back ratios are better than 10 dB.