The invention discloses a method and device for the approximation of pure lag, and the method comprises the steps: performing six consecutive first-order inertial inverse operations of an input signalby a preset first-order inertial inverse operation formula; solving the sum of a first output signal, a second output signal, a third output signal, a fourth output signal, a fifth output signal anda sixth output signal; performing a fifth-order inertial operation of a seventh output signal by a preset fifth-order inertial operation formula; taking an output signal of the subtraction operation as a ninth output signal, and performing four consecutive first-order inertial operations of the ninth output signal through a preset first-order inertial operation formula; solving the sum of a tenthoutput signal, an eleventh output signal, a twelfth output signal and a thirteenth output signal; multiplying the output signal of the subtraction operation by a preset second constant of 0.8333 to obtain an approximately pure lag output signal. The method solves a technical problem that a mode of approximating the pure lag through the only the first-order inertial link is larger in pure lag approximation error.