A new method for determining the grating parameters with the transmission spectrum and the optimized algorithm is described. In order to decrease the difference and ensure the precision of the calculation result, the symmetric-order efficiency ratio m=±1 is employed. Firstly, several points from the simulated experimental curve are selected, which is attained by adding the Gauss noises on the transmission spectrum, submit to the optimization methodNormal simple algorithm to calculation the optical parameters of the grating. The result is very near to the true value. Secondly, the volume phase holographic grating parameters are calculated with this metrology, the result is perfect, its relaive error is less than 2%. This demonstrated the reliability and the credibility of the metrology. It has some advantages such as no damage, facility of manipulation, repeat, easy generalized and low cost.