The invention discloses an online monitoring method for metering performance of a diaphragm gas meter (containing a diaphragm meter). A magnetic rotary table in an electro-mechanical transformation device is reasonably segmented, and movement information of each sub-segment is recorded and analyzed; qualitative and quantitative judgment of metering errors, inner leakage judgment, outer leakage judgment and safety judgment of a constant flow speed of the diaphragm gas meter are realized. When gas with one rotary volume is discharged by the diaphragm gas meter, a rotary shaft of a movement rotates for one circle; the magnetic rotary table is directly engaged with a gear of output of an in-meter rotary shaft or an out-meter rotary shaft, so that when the gas with one rotary volume is discharged by the diaphragm gas meter, the magnetic rotary table rotates for one circle; the metering resolution ratio of the gas meter is greatly improved; specific pertinent property matching is carried outon equipment and intelligent analysis and judgment are carried out on the movement information of sub-segment, so that the qualitative and quantitative judgment of the metering errors, the inner leakage judgment, the outer leakage judgment and the safety judgment of the constant flow speed of the diaphragm gas meter are realized, and reliable guarantees are provided for active safety management for gas; the online monitoring method disclosed by the invention has important meanings in the aspects of error management of the diaphragm gas meter, intelligent control of gas users and safety management of gas utilization.