DC-link capacitors, as some of the most vulnerable components in motor drive inverters, are crucial for online monitoring for aging. Their deterioration increases the probability of system failure. However, existing monitoring methods suffer from issues such as poor economic efficiency, high sampling frequencies, and interference with normal system operation. Therefore, a capacitor online monitoring method based on long-period transient signal analysis is proposed. This is used to estimate the equivalent series capacitance (ESC) of DC-link capacitors in motor drive inverters. Initially, a common-node admittance-capacitance equivalent transient model is established based on the system load switching process, and the characteristics of long-period transient signals is analyzed. Subsequently, an online monitoring model based on such signals is derived, and the criteria for initiating the monitoring process are determined. Then a polynomial reconstruction-based capacitor current baseline calibration method is proposed to eliminate the influence of sensor drift and to enhance monitoring accuracy. Finally, simulation and experiments show that the monitoring accuracy of the proposed method meets the requirements of capacitance monitoring.