A method of evaluating the truncation error of the effective bandwidth is proposed. Taking a trapezoidal wave as a signal mode and using MATLAB calculations，the energy truncation error and physical nature of the effective bandwidth formula 0.35 /tr for a non-sinusoidal wave are analyzed，and the disadvantages of the bandwidth formula 1 /τ for an ideal rectangular wave are explained. The analysis results show that the truncation error of the effective bandwidth formula 0.35 /tr is less than 3%，and the truncation error of the formula 1 /τ is less than 10%. Therefore，the effective bandwidth formula for the ideal rectangular wave is modified to be 3 /τ. The bandwidths of a signal channel and an oscilloscope should be chosen greater than the effective bandwidths of the measured signals.