The invention relates to a transmitting-receiving circuit system for receiving Beidou first-generation signals and Beidou second-generation signals. The transmitting-receiving circuit system comprises a receiving circuit, a data processing and system control circuit and a transmitting circuit and further comprises a fractional-N frequency synthesizer PLL, wherein the small-step frequency synthesizer PLL comprises a phase discriminator (PFD), a charge pump (CHP), a band-pass filter (BPF), a voltage-controlled oscillator (VCO) and a frequency divider which are annularly connected end to end, and further comprises a capacitor bank (Cap-Bank) and a Sigma-Delta modulator. The transmitting-receiving circuit system only adopts one fractional-N frequency synthesizer PLL, saves the area of an integrated circuit chip and reduces power consumption. The transmitting-receiving circuit system receives the Beidou first-generation signals and the Beidou second-generation signals through one receiving circuit, has the advantages of saving cost and being low in power consumption and high in using efficiency, integrates Beidou first-generation short message transmitting and receiving and Beidou second-generation high-accuracy positioning and facilitates market promotion.