A model for spectrum irradiance distribution with respect to the distance in the optical axis of underwater light sources was proposed based on the theory on light propagation and the attenuation of water on light. Model structure was built from physics and unknown parameters were determined by numerical methods. By utilizing the specific structure of the model, an identification approach was proposed where independent parameters were determined in different steps. The procedures for parameter identification were given in details. To validate the accuracy of the model and the feasibility of parameter identification, the model was further tested with underwater measurement data of an artificial light source. Results show that the model estimation is quite close to the measurement data and the root-mean-square value of the modeling error is only 4% of the measurement data, thus indicating high accuracy of the model. Application of this method can be found in modeling of underwater light field and underwater image processing.