The diamond nitrogen-vacancy center has become a very promising magnetic field sensor with its advantages of high sensitivity, sensitivity to external magnetic field, and detectability at room temperature. In this paper,we introduce the principle of magnetic measurement by nitrogen-vacancy center, and a magnetic measurement system based on nitrogen-vacancy center is designed and constructed, in which the noise is reduced to about 1/8 by the laser-fluorescence differential optical path, and the microwave mixing modulation method can enhance the slope of the ODMR spectral line by about 2.5 times, and the sensitivity of magnetic field measurement of 0.934 nT/Hz1/2 is finally realized based on the diamond samples with the concentration of nitrogen-vacancy center <300 ppb. Meanwhile, the NV magnetometry system based on dual microwave modulation and demodulation was designed, and a measurement uncertainty of 1.76×10-4 was achieved at a sampling rate of 10 kHz, and a measurement uncertainty of 10 ppm was achieved at a statistical time of 0.2 s based on the Allen variance method.