The Root sum of the square criterion (RSS criterion) is general accepted rule in pulse signal metrology and oscilloscope calibration. But the RSS criterion has a premise that the system should be two cascaded Gaussian system, otherwise the error is unacceptable. The paper derived the RSS criterion from the basic principle of measuring the pulse resource by oscilloscope or measuring the oscilloscope by resource. On the other hand, the characteristic of modern oscilloscope deviates Gaussian system greatly, and the premise of RSS are not satisfied anymore. The simulation and experiment data in the paper verified the premise of the RSS and indicated that the measurement result was unacceptable when the premise is not satisfied. The paper derived deconvolution algorithm in signal reconstruction and system identification to realize the metrology of the pulse signal and calibration of the oscilloscope.