The collisional-radiative model, that has been constructed for the helium plasma in the Sino-United Spherical Tokamak is introduced. The result of the 447.1nm (23P-43S)/492.2nm(21P-41D) and 492.2nm/504.8nm(21P-41S) line ratios is given. The line ratio method that is used for measuring electron temperature Te and density Ne parameters is described. The result from the line ratio method is consistent with the measurement of the 94 GHz interferometer. The method is also validated by comparing the relative population densities of the excited states which are deduced from the CR model and the measured intensity data of helium lines. The factors that brings errors into the diagnostic result are discussed, including the error from the measurements, and those from the uncertainties of the rate coefficients that are used in the CR model and the viewing chord integration characteristic of the optical emission measurement.