Abstract: To solve the problem of 1 fJ~1 pJ laser energy measurement, a femto-Joule level pulse laser energy measurement method based on integration of the time domain waveform was proposed, which used a photomultiplier tube (PMT) to get the response signal of femto-Joule level pulse laser. It realized femtoJoule level pulse laser energy measurement finally after amplifying, calibrating this weak response signal and the laser pulse time domain waveform integration. According to this method, a femto-Joule level pulse laser energy measurement device was designed and the measurement uncertainty of the device was analyzed. Experiments show that the device implements the energy measurement of pulsed laser source with 1 064 nm wavelength, 5 ns~1 s pulse width and 1 fJ~1 pJ energy range, the measurement uncertainty is 15.8%.