Hot bar soldering is a method used to connect electronic components. The stability of soldering horn temperature is the decisive factor of soldering quality. Due to the short time of hot bar soldering, the thermal inertia and random noise of thermocouple measurement have a great influence on the temperature control of the process. This paper developed a hot bar soldering power supply with STM32F407 microprocessor as the core, and designed the main circuit and control system of the power supply. By analyzing the delay response and time constant error of the thermocouple, this paper designed a new control method of hot bar soldering based on an Extended Kalman Filter (EKF) state observer, which realizes pulse width modulation and stable control of soldering temperature. It also analyzed the heating and thermal radiation effects of the heater tip, established a temperature model of the heater tip, and developed a simulation model of the hot bar soldering system based on the above main circuit and control scheme to verify the effectiveness of the control methods. A testing platform for the hot bar soldering system was built, and experiments were conducted according to the process parameters set by the simulation model. The simulated temperature waveform was compared and analyzed with the measured waveform. The results show that the trend of the simulated and tested temperature waveforms follows the same pattern. Compared to only using PID control, the control method based on EKF achieves a shorter adjustment time, reduces the impact of effective noise on the hot bar soldering system, and improves temperature control stability. The simulation model of the hot bar soldering system provides a reference model for the design of the hot bar soldering power supply. Finally, the hot bar soldering tests of FPC and PCB board, coaxial cable and LED circuit board were carried out to achieve the reliable connection of the components.