An optical generation scheme of microwave signals with multiple modulation formats based on a polarization modulator(PolM) and a Sagnac loop was proposed. The PolM was driven by a baseband coding signal to generate a polarization shift keying(PolSK) signal and the generation principle was theoretically analyzed. Two Mach-Zehnder modulators(MZM) were embedded in the Sagnac loop to modulate the PolSK signal transmitted clockwise or counterclockwise, respectively. The outputs of amplitude shift keying(ASK), frequency shift keying(FSK) and phase shift keying(PSK) microwave signals were achieved by properly adjusting the driving signals of two MZMs. In the simulation work, a 40 GHz ASK signal, a 20/40 GHz FSK signal and a 20 GHz PSK signal with a bit rate of 2 Gbit/s were produced. In addition, the broadband tunability of the bit rate and carrier frequency was verified. The system stability was improved with Sagnac ring structure. Furthermore, without changing the link configuration, the bit rate and carrier frequency for each modulation format of microwave signals can be tuned flexibly and independently by controlling the baseband coding signal and the RF driven signals of two MZMs.