Abstract: Modified three gradient method is a method to measure beam emittance developed from three gradient method which has been widely used in beam diagnostics. Beam radius' evolution versus focusing current can be fitted by a cross-over curve calculated from the K-V envelope equation through which the cross-over curve depends on beam radius, divergence in the initial plane and beam emittance. An approximation of the emittance can be got by the fitting program. In this paper beam radius under different focusing current was measured with Cerenkov radiation method; Time-resolved beam profile was recorded by a streak camera; A numerical fitting program, based on the K-V equation, was developed; Influence of beam radius and divergence in the initial plane and emittance on the cross-over curve were st