Resonant coupling is the technology widely applied for efficient wireless power transfer in medium ranges. This paper applies reflected load theory to analyze the multi-coil system consisting of several resonators. Different number of resonators would change resistance reflected to primary side and influence power delivered to load. To optimize power transmission of system, this paper purposes a method to calculate critical coupling point. A practical 4-coil system was implemented for verification. Based on this method, optimal 71.10 % of efficiency can be obtained by adjusting coupling between coils. Further, shifting frequency of source could enhance efficiency over 40 % when the distance d23 is less than 2 cm.