A coordinated platoon formation technique with a threshold is developed to coordinate the vehicle sequences to merge on the highway ramp. The merge threshold based on environmental parameters is optimized to reduce emissions, save time costs and fuel resources. First of all, this paper gives a threshold platoon formation scheme to overcome the complexity induced by the coexist of free vehicles and existed platoons. Then combining the vehicle kinematics and full expectation formula, the paper calculates the characteristic values related to the formation threshold:1)the likelihood of the platoon size,2)the expectation of time increments, and3)the expectation of platoon spacing. Secondly, the economic cost increment is analyzed according to the characteristic values, and the set of the driving constraint safety is derived by the transformation of the continuous dynamic state equation. Then the platoon formation optimization model based on the threshold is built, which maximizes the comprehensive economic benefits with the guarantee of safety. Finally, the proposed strategy is compared with the direct platoon formation technique in the simulation, which demonstrates that the proposed strategy can increase the comprehensive economic benefits and meet the real-time computing requirement. In addition, according to the simulation results, cruising distance and traffic density are the key factors affecting the comprehensive economic benefits of the platoon.