Considering the importance of start-up characteristics of the rubidium atomic clock in engineering applications, the objective of this paper is to optimize the start-up characteristics of the rubidium atomic clock by studying the theory of the rubidium atomic frequency standards, especially the light pumping process, and the effect of light intensity on frequency accuracy. Our analysis demonstrated that frequency accuracy is proportional to the light intensity, and hence, we propose a method for actively optimizing the start-up characteristics of the rubidium atomic clock by utilizing the fluctuations in light intensity. Additionally, some related experiments using the proposed method indicate that the light intensity–frequency coefficient of the rubidium atomic clock is improved from 1.84 × 10−9 to 4.21 × 10−10 V−1 within 30 min after the rubidium atomic clock is locked, and also, the lockout time is less than 5 min with a wide working temperature range (0–50 °C), indicating a significant improvement in the start-up characteristics of the rubidium atomic clock.