The "ultra-silent" characteristic of deep underground environment is essential for understanding the internal structure and evolution of the Earth. Jointly using the observations of a broadband seismometer and a gravimeter at the Huainan Deep Underground Laboratory (HDUL), and taking advantage of their frequency complementarity, surface and underground (-848 m) ambient noise levels in the ultra-wide frequency band (0.0001~50 Hz) are preliminarily investigated. Numerical results show that the underground ambient noise level is lower than the surface noise level in most bands, and the difference between them is smaller in the low-frequency band (0.0001~0.01 Hz) while larger in the high-frequency band (0.1~50 Hz). But for the frequency band 0.01~0.1 Hz, the underground ambient noise level is higher than the surface noise level, which is most possibly caused by air flow in the coal mine roadway. In addition, higher signal-to-noise ratio free oscillation normal modes are obtained in the underground roadway, which further demonstrates the "ultra-silent" characteristic of deep underground environment. Because the HDUL is in the stage of infrastructure construction, the "ultra-silent" characteristic of deep underground environment is expected to be more prominent with the improvement of observation conditions. The results provide valuable reference for the subsequent construction of the HDUL and for related geoscience studies.