The invention discloses a satellite-borne low-code-rate BPSK demodulation Costas loop circuit which comprises a multiplier, an active low-pass filter, a voltage-controlled crystal oscillator, a frequency divider, a detector, a comparator and a loop filter and is used for completing demodulation of an extremely-low-code-rate BPSK signal. The circuit is based on a classical Costas ring, wherein thecore circuits are all simulation devices, the phase discriminator is realized by a multiplier, the active low-pass filter is composed of an operational amplifier, the bandwidth of the active low-passfilter is extremely narrow, the voltage-controlled crystal oscillator outputs reference signals to be subjected to frequency division by a frequency divider to obtain BPSK carrier signals, and I-pathsignals and Q-path signals are subjected to phase discrimination by the multiplier and then can obtain locking indication signals through a comparator. Aiming at BPSK signal demodulation with an extremely low code rate, the circuit can provide an excellent demodulation performance and a reliable locking indication signal, meets the use requirements of satellites, and has the advantages of simplicity, reliability, easiness in implementation, low cost, good radiation resistance and the like.