<p>This paper studies the change of the busbar joint resistance which is closely related to service life of the bus. By means of thermodynamics, the forming mechanism of busbar joint resistance is further analyzed and the theoretical formula of the resistance is derived through importing the idea of contact area ratio. Then, the evolution of the resistance over using time is also studied under normal conditions and it is used to explain the dramatic variation in service life of the bus. Finally, an online detection model for busbar joint resistance based on LS-SVM (least square support vector machine) is established by analyzing the heat transfer principle and considering the heat generated from electrifying the busbar joint, which serves as the heat source. Experimental results show that the working state of busbar joint can be monitored online effectively by the detection model to avoid the loss due to high resistance.</p>