The invention belongs to the technical field of phenolic resin production, and particularly relates to a preparation method of castor oil modified linear phenolic resin for a copper clad plate. Phenolic resin is modified with castor oil by using an acid catalyst, and linear thermoplastic resin is prepared. The resin after curing has high crosslinking density and few low molecular volatiles and cansignificantly improve the toughness and heat resistance of the copper clad plate when being used for preparing the copper clad plate, the resin can also be used for the lead-free welding process, theservice life of the copper clad plate can be prolonged, and the production cost can be reduced.