Abstract: The use of composite materials in low-temperature fuel tanks is an effective approach to achieve weight reduction and enhance carrying capacity for launch vehicle structures. However, as the tank is subjected to both ultra-low temperature cycling and mechanical loads during service, it is prone to internal microcrack formation in composite materials, posing a threat to structural safety. In this study, a simple pre-stressing device was used to conduct ultra-low temperature cycling tests on carbon fiber/epoxy orthotropic laminates. The research focused on the initiation and evolution of microcracks in composite laminates under the combined action of pre-stress and ultra-low temperature cycling. The results indicate that the microcrack density at the edge layers of the laminate is generally higher than that in the inner layers, but the central two 90° stacked layers exhibits the maximum microcrack density. Compared to conditions of only low-temperature cycling, the pre-stress results in a higher microcrack density in the laminate under the same number of low-temperature cycles, with a faster growth rate. With an increase in the number of cycles, the growth rate of microcrack density initially accelerates and then slows down, eventually reaching saturation. As the pre-stress level increases, the initiation and propagation rates of microcracks in the laminate are further intensified. This study provides a preliminary insight into the mechanism of microcrack initiation and evolution in composite materials under the coupled action of load and ultra-low temperature cycling, offering meaningful references for the development and application of low-temperature composite material tanks.