The invention discloses a density measuring instrument and an application method thereof. The density measuring instrument comprises a pressure sensor, a weighing grain storage chamber, a switch and a counter. A probe of the pressure sensor is located outside the density measuring instrument, and the pressure sensor is used for measuring external pressure; the weighing grain storage chamber is used for storing weighing grains, an outlet is formed in the bottom of the weighing grain storage chamber, the interior of the weighing grain storage chamber is communicated with the exterior of the density measuring instrument through the outlet, and the weighing grains can pass through the outlet; the switch is used for controlling the outlet in the bottom of the weighing grain storage chamber to be opened or closed; the counter is used for computing the number of opening-closing times. By means of the density measuring instrument, the parameters including pressure and density of the pressure gradient can be obtained and determined, and therefore the pressure gradient can be obtained.