A calibration circuit for BESIII TOF electronics is described in this paper.An exponential-decay calibrating signal

of which the charge(integrated area of amplitude) was set by remotely-controlled DAC via VME bus

was generated by fast analog switch and fed to each time-and-charge measurement channel of TOF readout electronics.It has the charge accuracy of better than 10 bits and the dynamic range of more than 20 times.Detailed tests suggest that the nonlinearity of charge measurement

normally about 2%

can be greatly reduced to less than 0.6% after calibration.This calibration circuit scheme has been applied in BESIII project successfully.