Objective To understand the process of making treatment decisions for cancer patients by their family members, to analyze the influencing factors, and to provide reference for assisting them to make treatment decision. Methods Fourteen family members of cancer patients received in-depth interviews, then data were transcribed and analyzed by employing grounded theory to form categories and core categories. Results One core category of inner mechanism of decision-making for cancer treatment by family members was identified, including 6 categories as endogenous trust, information collection, rationalization of decision, decision generation, facilitators and barriers. Conclusion Decision making for cancer treatment by family members is a multiple-step process and influenced by various factors. Medical staff should understand the inner mechanism of treatment decision making of family members of cancer patients, thus to assist them to make decisions that meet the actual needs of patients.