The invention discloses a method for testing the cementing strength of a cement first cementing surface of an oil well. The method comprises the steps of: (1) making a simulated casing-cement ring combined structure sample; (2) vertically placing the simulated casing-cement ring combined structure sample, and clamping the two ends on a hydraulic tensile testing machine; starting the testing machine, loading the sample slowly and uniformly to obtain a yield load value F; stopping loading immediately after the sample is broken, obtaining the load value at this time which is the maximum load value F; reading the extension length L of a yield stage on a force-extension graph; calculating the cementing strength between a cement stone and the parallel portion of a metal tensile rod, that is, the cementing strength of the cement first cementing surface. The method for testing the cementing strength of the cement first cementing surface of the oil well is reliable in principle, easy and convenient to operate, can simultaneously test the cementing strength and the tensile strength of the cement stone, optimizes the cement slurry formula, obtains a cement slurry system meeting on-site construction requirements, and has broad market application prospect.