Atomic magnetic field measurement technology is gradually developing into a new generation of ultra-sensitive magnetic field measurement technology with the progress of quantum sensing, information and instrumentation technology. The spin-exchange relaxation free (SERF) atomic magnetometer is a kind of magnetometer with extremely high sensitivity. A single beam atomic magnetometer based on magnetic field modulation is proposed in this paper. The transmitted light intensity of alkali metal atom gas chamber was detected to obtain the corresponding external magnetic field. The rubidium atoms are pumped by near-infrared light with a wavelength of 795 nm. The maximum sensitivity of the system is 175.4 fT/Hz^1/2 with the modulation frequency of 1.1 kHz, the modulation amplitude of 82 nT, and the modulation index of 0.887. Experiments show that the designed structure is reasonable and easy to miniaturize, and has practical value in many fields such as weak magnetic field and biological magnetic field measurement.