The present invention provides a calibrating device for a low speed current meter, which comprises an adjustable bracket, and an annular water tank, a rotary column, a rotary arm, a stepper motor, a speed reducer, and a synchronous cog belt which are arranged on the adjustable bracket. A partitioning board is arranged inside the annular water tank. The rotary arm comprises a beam, balanced code shelves which are respectively arranged on two ends of the beam, and an adjustable probe clip with a probe, and the rotary arm is externally covered by a decorative hood. The rotary column is arranged in a center of the annular water tank. The upper part of the water tank is provided with a sealing chamber. An annular four-groove mercury cup, four contact needles and a bridge circuit are arranged inside the water tank, wherein the contact needles are driven by a rotating shaft, and the bridge circuit is formed for the power supply input and the flow velocity signal output during the rotational operation of the probe. A base at the lower part of the rotary column is provided with an air level. The center of the rotary column is provided with the rotating shaft supported by a bearing. A driving disk on the rotating shaft is connected to the stepper motor through a synchronous cog belt and the speed reducer. The stepper motor acquires the distribution and the control of a driving power supply through a computer. The device is small in volume, portable, convenient to use, high in precision, and particularly suitable for the calibration of the flow velocity not larger than 40cm/s.