The invention relates to a diagnostic method of an engine variable intake manifold execution mechanism. The method comprises the following steps that the intake manifold pressure values of an intake manifold under different states are calibrated; the pressure values calibrated in the step a serve as expectancy values to be stored in an ECU; a real-time intake manifold pressure numerical value is obtained; the intake manifold is in the calibrated required state; the actual air inlet pressure is applied to be compared with the calibrated air inlet pressure; fault judgment is carried out, wherein two pressure values in the step f are compared, after the difference value exceeds a numerical value range, timing begins to be carried out, after set time is up, if the actual difference value of the two pressure values is still out of the set numerical value range, it can be confirmed that the execution mechanism fails; or, before the set time arrives, if the actual difference value of the two pressure values is reduced to be in the set numerical value range, timing is stopped, and timing begins to be carried out again until the next difference value is larger than the set numerical value range; and the diagnostic function is completed, and diagnosis is stopped. According to the method, diagnosis can be completed without the mechanism, and diagnosis is rapid and accurate.