Abstract: Porosity is a fundamental physical parameter, which has a significant impact on temperature, humidity, and biological field of bulk grain. A study of digital image method was carried to investigate porosity of bulk wheat grain under different overburden pressures. The tested material was wheat produced in Henan province, China. It has a natural bulk density of 793 kg/m³, moisture content of 10.9%, and the average particle diameter of 4.6 mm. The bulk wheat was tested in a new designed compression apparatus for bulk grain, which has a dimension of 120 mm×120 mm×50 mm. The height of the compression apparatus is larger than 10 times of the wheat particle, and complies with the requirement of compression test. The pressure applied on bulk wheat specimen was 4, 7, 15, 30, 59, 119, 178, 237, 296, and 333 kPa, the maximum pressure of which is considered as the maximum pressure in large grain bins. The digital image was collected through CMOS lens, a DC regulated light source was adopted with an illuminance of lux, and a procedure compiled by MATH WORKS was used to revise the distortion of the image. The digital images of bulk wheat were collected under different overburden pressures, the space of void and wheat was distinguished by the gray level of the images, then grain particle and void was segmented through OTSU threshold method, and the porosity of bulk wheat was defined by picture element quantity of void divided by the total picture element quantity. The threshold pixel point value of bulk wheat digital image was investigated, and the results showed that, when the threshold pixel point value was 71, the determined porosity was consistent with the test result. Bulk grain porosity was also calculated by consolidation theory, which considers the wheat kernel as a non-compressible material. The digital image method was verified with the theoretical method. The results showed that:the porosity of bulk wheat grain decreased with the increase of overburden pressure, and the relation between porosity and overburden pressure can be expressed by the power function. The coefficients of a and b were -0.006 89 and 0.424 9, respectively, in theoretical method, and the coefficients of a and b were -0.000 46 and 0.840 2, respectively, in digital image method. Porosity calculated by theoretical method was consistent with digital image method. Therefore, digital image method can be used to determine the porosity of bulk grain under different overburden pressure, temperature and humidity fields without sampling or disturbing the material. The results can provide parameters for numerical simulation of grain in bins.