An experimental LaNi5 alloy’s hydrogen absorption/desorption system was established to explore the strain generation process mechanism on the reactor wall during the cyclic hydrogen absorption/desorption process of LaNi5 alloy powders at room temperature after different cycles and periods. On this basis, the influence of the hydrogen pressure on the wall strain of the reactor was verified. Results showed that with the increase in cycles, the reactor wall was subjected to an expansion strain, and the closer to the reactor bottom, the greater the strain value. As the hydrogen pressure grew from 3.0 to 4.0 MPa, the strain value at the reactor bottom increased obviously. Still, it did not grow during cycling experiments with hydrogen, indicating that the agglomeration phenomenon was mainly attributed to the pulverization effect after hydrogen absorption by the alloy instead of the gas pressure itself. After 24 cyclic reactions, the wall strain at the 1/5 position from the bottom experienced changes in growth laws since the thermal strain was not enough to influence the expansion strain triggered by agglomeration, and the closer to the reactor top, the more greatly the wall strain was influenced by reaction heat.