In the laser communication and imaging system, whether the Cassegrain type optical antenna is used as a laser signal transceiver or an optical imaging device, the existence of aberrations will inevitably influence its signal intensity and imaging quality. By analyzing optical antenna of the third-order aberration theory and the perturbation theory, according to the relationship between the antenna gain factor and the occlusion ratio, a convenient and fast method for solving the Cassegrain type optical antenna structure was proposed. This method can not only consider the efficiency of the optical antenna, but also meet the requirements of the volume size and aberration of the optical system design. Especially in the case that only knowing the focal length of the optical system, the distance between the primary and secondary mirrors, the occlusion ratio, the structural parameters of the optical antenna can be calculated quickly. Not only gives the calculation formulas of the optical structure parameters for four types Cassegrain optical antennas, but also explains the advantages and disadvantages in the different application. According to comparing the calculation results of four typical optical antennas with the optimal results, the results show that this method is a convenient, fast and accurate method to solve the Cassegrain type optical antenna structure, and has practical significance in engineering.