A spectrum optimization problem of maximizing the mesopic visual luminous efficiency of phosphor-coated white light-emitting diodes (LED) by exciting green and orange phosphors with a blue LED. The objective function is to maximize the average mesopic visual luminous efficiency at multiple brightness levels. The optimization model for correlated color temperature (CCT) is discrete and exhaustive, and it can only maximize the average mesopic visual luminous efficiency for varied brightness levels under a constant color temperature. A nonlinear constrained optimization model with a simple objective function is suggested in this study to attain the maximum mesopic visual luminous efficiency at varied brightness levels, and the CCT is considered an optimization parameter varying in a specific range. The experimental results show that the model can obtain the optimal LED spectrum, while meeting the requirements of color rendering index, color fidelity, and a given mesopic visual brightness level, and that it truly achieves the purpose of maximizing the medium visual luminous efficiency under the specified brightness level.