An air chamber is introduced which is made of two rectangular prisms. Stability and sensitivity of the chamber are improved using the reflectivity character and senseless to tilting of light beam. Transmission characteristics of light beam are analyzed. Transmission equation of probing light beam and measurement equation of the system at both single-wavelength and dual-wavelength are put forward. The times N of light beam going back and forth in the chamber is determined by distance of between planes of symmetry through respective apexes of the prisms. The sensitivity may be chosen by means of adjusting the distance. The results of experiment indicate that the sensitivity of the photometric sensor is N times that of single-pass cell at lower concentration of material to be measured, and when concentration of material is higher, the sensitivity is proportional to the times of light beam going back and forth at the smaller N, and that the times of light beam going back and forth multiplying action on the sensitivity gets weaker at the greater N. The photometric sensor based on rectangular prism with optical fiber is suitable to measuring trace material.