We propose and experimentally demonstrate an RF signal multiplier based on harmonics injection locking to a single mode Fabry-Pérot laser diode (SMFP-LD) followed by an optoelectronic oscillator (OEO). The injected beam is generated by a tunable laser, which is modulated with a frequency fm generated by a microwave source. Then the modulated beam is optically injected to the SMFP-LD with a negative wavelength detuning ▵λ, where one of the harmonics of the injected beam is locked to the corresponding mode of the SMFP-LD. Hence, due to the power gain of the SMFP-LD to the optical harmonics of the injected beam, stable electric harmonics of the modulated frequency with high signal to noise ratio (SNR) are generated by beating the output of the SMFP-LD after optical injection. In order to obtain one of the generated electric harmonics signals, we employ an OEO feedback loop to optimize the signal with high purity, which is determined by the electrical bandpass filter (EBPF) and the low noise amplifier (LNF) in the OEO loop. As the gain of the OEO loop is higher than the loss, a high-purity oscillating signal can be generated. In the experiment, we generate a sextuple signal (20 GHz) of the modulated frequency with the SNR and phase noise of 51.14 dB and -102.44 dBc/Hz@10kHz, respectively. Compared with other photonics structure to generate multiple frequency, the proposed method shows a simple structure, high SNR, easy to operate and low phase noise.