The invention provides a multi-mode small low phase noise broadband dot frequency synthetic circuit, which comprises a radio frequency amplifier, a power divider, a low pass filter, a phase discriminator, a switching circuit, a first loop filter, a second loop filter, a voltage-controlled oscillator, a coupler, a radio frequency amplifier, a band-pass filter, a programmable frequency divider, a programmable logic controller, an interface circuit and a power conversion module. According to the multi-mode small low phase noise broadband dot frequency synthetic circuit provided by the invention, the frequency point output with the minimum 1 MHz step within a fixed broadband of 3.5 GHz to 6 GHz can be controlled through an SPI (Serial Peripheral Interface), and the frequency point output is used as the single frequency point zero control output; the output of eight different frequency points at most can be realized under a switching mode through switching; meanwhile, the phase noise indicator at 4.8 GHz is -110 dBc/Hz@10 kHz, the nonharmonic stray indicator is -75 dBc, and the module volume is only 60 mm x 65 mm x 15 mm.