Library reading room is a crowded building with large thermal and moisture load. Design of freezing dehumidification and radiant air-conditioning system for the reading room should achieve the target of non-dewfall condition of the radiant panels, low energy consumption and optimal comfort. For this purpose, the indoor design temperature and humidity of the room were figured out mostly according to predicted mean vote. Meanwhile，in order to lower the energy consumption, air-conditioning load should be rationally allocated between dehumidifier unit and radiant panels. And a method of calculation aided by quasi thermal- humidity ratio line approximation was discussed. Through the approximation calculation, considerable accurate design parameters can be obtained. By an analysis on a design example of a library reading room, the efficiency of the method was verified.