Using the modified transport approximation，the authors calculated and obtained 18-group neutron constants. In this paper，the authors try to examine the validity of these 18-group neutron constants by calculating the Tritium production rate of fast neutron in LID spheres. The modified transport approximation equation and its matched 18-group neutron constants are used to calculate the Tritium production rate of fast neutrons in two different models. The results are in good agreement with those obtaind by other authors using different transport equation and group constants，the relative errors are within 3%. The calculatied results and the comparison show that the modified transport approximation equation and its matched 18-group neutron constants can be used to describe the transport behavior of fast neutrons in light material，and the 18-group neutron constants obtained by the authors are reliable and rational.