The invention provides a photovoltaic monitoring system and a protocol conversion method when a large number of devices access. According to the method, for a photovoltaic array corresponding to each communication manager, addresses of devices in the photovoltaic array are planned into a device address setting range; the device address setting range comprises independent address ranges of the devices; device information serial numbers are planned into a device information serial number setting range; the device information serial number setting range comprises independent serial number setting ranges of the device information; and the device addresses and the device information serial numbers are combined and are mapped into a protocol information body address. According to the method, the limitation to the device addresses under the condition that the large number of devices access is solved; the allocation of the device addresses of the photovoltaic arrays is simplified; and the device addresses are identified accurately.