The attenuation of light was a common result that the light was absorbed and scattered by the artificial fog particles when it transmitted in the artificial fog. The absorbing attenuation process of light transmission in the artificial fog was that the artificial fog converting incident light energy into other forms of internal energy (such as heat energy). The scattering attenuation process of light transmission in the artificial fog was that the artificial fog particles intercepting incident radiation energy to form infrasonic waves and to radiate peripherally so that the incident light energy was reduced on the original direction of transmission. The mechanism of light transmission attenuation in the artificial fog was analyzed. The formation method of the artificial fog was expounded and the measuring principle of the artificial fog transmittance was described. A simple and reliable measurement method of the optical transmittance of the artificial fog in the fog chamber was proposed. The optical transmittance measurement system of the artificial fog was built by using incandescent lamp, power with steady current and voltage, lens, selenium photocell, micro-galvanometer, optical bench, hygrothermograph, humidifier, etc. Under different conditions of humidity, the optical transmittance of the artificial fog was obtained on the basis of measuring the photocurrent before the fog was formed in the fog chamber. The test results show that the measurement system is stable and reliable. During the 43 minutes after the artificial fog was formed, the optical transmittance of the artificial fog was averagely less than 5 percent and the optical transmittance increased gradually with the extension of time. In addition, the optical transmittance of artificial fog didn’t produce obvious change while air humidity increased from 68.7% to 85%. The measurement system can be used to measure transmittance of smoke screen, water mist and other aerosol.