The invention relates to an RCS (radar cross section) measurement device based on a single continuous terahertz laser source and belongs to the technical field of terahertz RCS measurement. The RCS measurement device can be used for solving the problem in measuring the RCS of target radars of different sizes in a 2.52THz waveband. The RCS measurement device comprises a CO2 laser pumping continuous terahertz laser, a chopper, a beam splitter, an electrical control variable expansion ratio device, a P3 off-axis parabolic mirror, a P2 off-axis parabolic mirror, a precise movement platform, an M1total reflector, a P1 off-axis parabolic mirror, a scattered signal detector, a first lock-in amplifier module, a computer and a chopper driver. The RCS measurement device is suitable for RCS measurement on the target radars of different sizes.