In view of the shortcomings of the existing methods for detecting the internal resistance of batteries, such as poor accuracy, high complexity and affecting the life of the battery, an online detection system of internal resistance based on pulse current discharge method is studied. The pulse current is discharged to the load through controlling the MOSFET, and acquiring the voltages and current before and after discharge. The AC differential circuit amplifies these signals and then transform into DC signal. After the filter circuit processing, they are converted into the digital signals. Finally, the MSP430 microcontroller analyzes and calculates the internal resistance of the battery online. The experimental results show that the pulse current discharge method needs to less parameters, and reduces the complexity of the system, improves the accuracy of online measurement of battery internal resistance, at the same time avoids the damage to large transient current on the battery and prolongs the service life.