The design and principle of reflective trap detectors are discussed which consist of three windowless Hamamatsu S1337-11 silicon photodiodes. Linearity, spatial uniformity, polarization sensitivity and stability of the trap detectors were measured. By calibration against a CryoRad-Ⅱ cryogenic radiometer, absolute spectral responsivity of the trap detectors was obtained between 488 nm～786 nm with uncertainty less than 0.035%.