The invention relates to a rectangular 100-250A electric fire alarm detector, which mainly solves the technical problem in the prior art that a current transformer and a main body monitoring control part cannot be freely combined, are separately mounted and connected with each other by an outer connecting wire. The detector comprises a freely combined module, a circuit board and the current transformer. The freely combined module comprises a limiting piece, a spring, a cover, a plug, a circuit board, a base, a gasket, a bolt and a round pin. The circuit board is provided with the control circuit comprising a power supply and a control part. The control part mainly comprises an MCU (microprogrammed control unit) main control part, an A/D (analog/digital) collecting and amplifying part, a reference source part, an EEPROM storing part, a communicating part, a display screen key part, a state indicator part and a debugging interface part. The rectangular 100-250A electric fire alarm detector adopts the round pin as the fixing mode between the freely combined module and the current transformer for limiting and locking without fixing by a fastener, thus being fast in mounting and saving labor and time.