Cyber-physical system (CPS) provides more powerful service through combining software service and physical device. It is an effective solution to organize various CPS services to realize collaborative decision making (CDM). In CPS, finding out the most competent participators for CDM sponsor is a core problem. To solve this problem, we propose a novel capacity and trust computation based CPS service selection mechanism in intelligent and automatic manners. It comprises three phases, including capacity evaluation, trust computation and negotiation selection. In the first phase, CDM sponsor describes formal semantic of decision task and computes the capacity evaluation values according to participator instructions. In the second phase, we design a novel trust computation method to calculate the values of activity trust, subjective belief, objective reputation, physical trust and recommended trust respectively. In the third phase, service selection is achieved through a negotiation mechanism according to capacity evaluation and trust computation.