Disclosed are a colorimeter and a reflectivity measuring method based on a multichannel spectrum. The colorimeter includes a main unit and a calibration box, wherein the main unit includes an integrating sphere, a light source and a main sensor, a detection hole is formed in one side of a top of the integrating sphere, a light-through hole is formed in a side of the integrating sphere, and a measuring port is formed in a bottom of the integrating sphere, the light source is arranged outside the light-through hole, and the main sensor is arranged outside the detection hole; the calibration box includes a housing and a white board arranged at a top of the housing, the white board is correspondingly matched with the measuring port, and the calibration box is connected with the main unit; the sensor is a multichannel spectral sensor.