Underwater optical imaging is an important underwater measurement method. The existing underwater camera imaging resolution measurement method is affected by the water body and the measurement method, and it is difficult to accurately measure the imaging resolution. The underwater camera imaging resolution measurement technology based on an underwater collimator is proposed to directly measure the underwater camera imaging resolution by generating parallel light beams in the water. Through simulation, the modulation transfer function (MTF) of the underwater collimator in visible light in water and single wavelength in the air are basically the same. In conclusion, a method for adjusting the underwater collimator in the air is proposed. The experimental test is carried out for an underwater camera developed in the laboratory, and its resolution under the condition of visible light in water and a 635 nm light source in the air is the same. The experimental results show that the proposed underwater camera imaging resolution measurement method based on the underwater collimator can effectively eliminate the influence of water on the measurement and realize the accurate measurement of the underwater camera imaging resolution.