The invention relates to a multi-purpose underground observing room, belonging to the technical field of soil monitoring. According to the invention, a plurality of transparent tempered glass observation windows are arranged on the side wall of the multi-purpose underground observing room, provided with different observation devices, and directly communicated with an external soil body, and a plurality of round holes are respectively formed in transparent tempered glass of the observation windows, and are provided with detection sensors or sampling heads connected with the sampling devices. According to the invention, the transparent tempered glass is adopted, and the observation devices are placed in the underground observing room, so that the moisture, temperature, conductivity, pH and ion change of soil at different depth can be accurately monitored, and meanwhile, the color change, the form change, the interior physical property change and the interior chemical property change, as well as the change of the soil physicochemical property and the crop root morphology change above and below a straw layer during the decomposition process of straw can be observed; and the multi-purpose underground observing room has the advantages of convenience in observation and large quantity of measurement items.