The invention aims to provide a foaming agent with low cost and low ODP value and a preparation method of the rigid foam plastic that adopts the foaming agent. The foaming agent is composed of fluorodichloromethane the weight percentage of which occupies 5 percent to 95 percent and dimethyl enther the weight percentage of which occupies 5 percent to 95 percent. Because the ODP value of the dimethyl enther is zero, the addition of the dimethyl enther can reduce the dosage of HCFC-141b, and the ODP value of the foaming agent is obviously reduced, which is favorable for protecting the environment. In addition, after being mixed according to a certain proportion, the two chemical compounds can be used as the foaming agent, the comprehensive performance can be obviously improved, the combustibleness can be reduced and even can not generate, the coefficient of thermal conductivity of the mixed gas is feasible, and the stability of the size of the made foam can be improved. The preparation method of the rigid foam plastic has the advantages that the raw material comprises polyisocyanic ester and polyatomic alcohol, and the adopted foaming agent is composed of HCFC-141b the weight percentage of which occupies 5 percent to 95 percent and the dimethyl enther the weight percentage of which occupies 5 percent to 95 percent.