The experiment of optical-microwave double-resonance in trapped mercury is the basis for the research of mercury ion microwave frequency standard. The second-order Zeeman effect is one of the important factor affecting frequency stability and the magnetic field effect. The Zeeman spectrum of ground state of trapped 199Hg+ ion was measured by the method of optical-microwave double-resonance. The clock transition frequency and linewidth of the clock transition line were obtained by scanning the microwave frequency in a narrow region. The magnetic field in trapping area was shielded by the materials of permalloy. The linewidth of the clock transition line is decreased to about 1.5 Hz. Comparing the clock transition frequency without magnetic shielding, the clock transition frequency of magnetic shielding decreases 13.5 Hz. According to the second-order Zeeman effect, it’s calculated that the magnetic field of trapped region is about 0.1 Guass.