This paper studies the improved solution and nonlinear isolation performance evaluation of crank train inerters (CTIs). First, the improved solution of the single-degree-of-freedom system with CTI is obtained, which is a modification of the analytical solution obtained by the harmonic balance method. The parameter analysis and nonlinear dynamic characteristics of the CTI system are studied. The stiffness softening phenomenon occurs in the CTI system, and the nonlinear phenomenon is proportional to the excitation amplitude. The absolute transmissibility of the CTI system is derived, and parameter analysis and linearization design are carried out. The vibration isolation performance of CTIs is better than that of linear inerter systems, and the advantage is more obvious for a large excitation amplitude. For seismic excitations, the nonlinearity of the CTI is beneficial for isolation, which is better than a linear inerter. The isolation effect is inversely proportional to the crank length and proportional to the excitation amplitude.