In the measurement of gas concentration by means of the light extinction, two identical rectangular prisms are used to increase the optical path of transmitted light in the gas and the detection sensitivity of the gas cell is improved by making use of total internal reflection of prisms. Focusing of the paraxial light beam by means of convex lens solves the problem that output beam moves in the second measurement. Transmission equation of probing light beam and the sensitivity of the system are proposed in the condition of the paraxial light beam, and simulation analysis is made for the sensitivity. Based on the measurement method of single wavelength, the gas concentration and system error are calculated and analyzed respectively.