The influence of the parameters on operating stability of a passenger car-trailer was analyzed by constructing the mathematical model of the passenger car-trailer. The root-locus curves were drawn in Matlab environment. The results show that the velocity has a bigger influence on operating stability of passenger car-trailer; the stability margin of the car is decreased; and different parameters have different influences on the stability so that select the right structure is propitious to improving operating stability of a passenger car-trailer.