A multimeter function expander used with a multimeter to measure electrical variables. The casing adopts a wiring board with an AC power input plug and an AC power output socket. A current transformer is used for sampling the measured AC current. The primary coil of the transformer is connected in series between the AC power input and output. The secondary coil is connected in parallel with a toggle limit switch and n shunt resistors. The current limiting resistor and the current limiting trimpot are connected in series and then connected in parallel for alternating current. The voltage reverse protection diode is connected in series with the rectifier diode to output the test signal. The expander has the advantages of easy operation, safety and reliability, wide measurement range, high test accuracy and wide application range.