A novel formulation, surface current measured equation of invariance (SCMEI), for the on-surface measured equation of invariance method (OSMEI) is deduced from the reaction integral equation. This formulation directly couples the current densities and scattered tangential electrical fields on the perfect conducting surface in the MEI equations. Compared with the original OSMEI, the advantage of the SCMEI is that when finding the MEI coefficients, only measuring functions of electric fields need to be integrated from the selected metrons, and the surface current can be directly obtained from the solution of the MEI equations. The computing burden for the measuring functions is thus reduced. The SCMEI can be used to analyze open boundaries, such as strips and wire antennas, etc., while the OSMEI was applicable only to closed boundaries because of the requirement of simultaneous integrals of electric and magnetic fields. The limitation for the present achievement, which needs to be further improved, is that the number of the nodes coupled in the SCMEI equations should be larger than that of the general OSMEI equations. The validity of this formulation is verified by modeling scattering of perfect conducting cylinders and radiation of thin-wire antennas.