The invention discloses a method simultaneously measuring a reflection step and Tyvek reflectivity of light in a detector. The method comprises steps that 1), a selected cylindrical detector of which an inner wall utilizes a high reflection Tyvek material is filled with pure air, and a photomultiplier is arranged; 2), a selected light source is utilized to irradiate in the detector, and light signals are acquired by the photomultiplier; 3), a reflection area of the detector inner wall is changed, and the step 2) is repeated; then fitting is carried out according to the acquired light signals, valid attenuation length lambda' of photons in the detector is acquired when reflection areas are different; and 4), according to data acquired in the step 3), a formula 1/lambda'=-ln(f)/L-ln(r)/L is utilized for fitting of a corresponding point of the lambda' with respect to a proportion r of the reflection area to the detector inner wall area, and Tyvek reflectivity in the air and the average reflection step are further measured. The method has advantages of simple operation, small error, high precision, novel principle and strong application values.