This paper presents the design and fabrication of a weak magnetic field measurement instrument based on optical pumping magnetic resonance frequency-amplitude response. The instrument employs a sweep frequency method for measuring weak magnetic fields, offering a simple and intuitive approach with a wide measurement spectrum. A phase-locked amplifier is used to implement the sweep frequency, with the sweep frequency interval set to drive the coil and generate the sweep magnetic field. The AD module of the DSP28335 development board is utilized to achieve signal phase-locked amplification and data acquisition/analysis. A PID temperature control system was designed using the DSP28335 and hardware components, with a reasonable structural design for the apparatus. Analysis of the experimental results showed a small relative measurement error, and the apparatus is easy to operate and adjust. This system can stably measure weak magnetic fields at the μT level, and can basically meet the teaching requirements for weak magnetic field measurement experiments in university physics courses.