A working method of a modular uninterruptible power supply (UPS) includes: obtaining working parameters of the modular UPS, where the working parameters include an input voltage parameter, a load parameter, and a battery parameter; and adjusting a working mode of a power module in the modular UPS according to at least one of the working parameters of the modular UPS, so that not all power modules are in a same working mode, where the modular UPS includes K working modules, and 2≤K.