As energy and environmental problems worsen on a global scale, enthusiasm has grown for the use of low-grade thermal energy conversion (LTEC), which utilizes unused low-temperature heat sources. The majority of these unused low-temperature heat sources are difficult to recover owing to their low temperature; thus, the thermal efficiency of the cycle in LTEC is, in principle, smaller than for the standard power generation cycle. Therefore, improving the performance of the LTEC system by using a multistage heat engine to mitigate the irreversible loss in heat exchange has often been studied. However, little is known about the properties of this system and its effectiveness. This study used a multistage heat engine to analyze the maximum power usable by LTEC and proposed an evaluation formula based on the maximum power. The evaluation formula derived for the maximum power of a multistage heat engine revealed that, compared with a single-stage engine, the power increases approximately twofold at its maximum, and that the thermal efficiency of the cycle for the maximum power is constant, regardless of the number of stages.