Abstract: When drilling bit has cut through strata, crushed rocks and gases are carried to ground by drilling fluid. Some of them are dissipated in the air immediately, and some are dissolved in drilling fluid. To detect the dissolved volume and volume fraction of hydrocarbon gas components is one of the most important methods to discover and evaluate oil-and-gas shows. Different hydrocarbon gas components have different dissolving capacities. And reservoir cannot be discovered and evaluated timely and accurately if the dissolving test of various hydrocarbon gases is just done. For this reason, a “dissolving coefficient” and its solving method are proposed for the first time. Through the dissolving test of various hydrocarbon gases with a rotary simulation device of drilling fluid in high temperature and high pressure, we recognize the dissolving coefficient in these circumstances. According to the dissolving coefficient, the true content of a certain hydrocarbon composition is calculated. This will be advantageous to discover and evaluate oil-and-gas shows timely and accurately.