Abstract: A good understanding of flow characteristics and drawing mechanism of loose top coal is an important foun- dation for optimizing the drawing parameters,increasing the top coal recovery and reducing the rock mixed ratio in longwall top coal caving. The proportion of top coal in different sizes is called size distribution,and size distribution is one of important factors affecting the flow characteristics of loose top coal. In order to study the effect of size distribu- tion on the flow characteristics of loose top coal,both physical simulation experiments and theoretical analysis were conducted based on a self-developed experimental device. The initial drawing volume,the flowing path of top coal and the evolution characteristics of initial boundary of top coal were investigated in depth. The results show that under the different size distribution of top coal,the initial drawing volume increases with the increase of average size of top coal. The size distribution of top coal draw out changes significantly,and when the proportion of small top coal is higher,the large top coal is easier to be drawn. In contrast,the large top coal is more difficult to be drawn due to the frequent for- mation of arch structures. The lowest point of the initial boundary of top coal is gradually moved toward the gob side other than the center line of the shield opening during the top coal drawing process. The moving path of the lowest point is basically consistent with the horizontal parabolic form,and the larger the average top coal size is,the more ob- vious the characteristic is. The speed of top coal particles above the caving support is obviously faster than that of top coal particles near the gob side. When the average size of top coal is small,the speed is faster and the number of arc- hing frequency is relatively low. After the top coal passes through the shield opening,the diffusion angle of top coal flow increases with the increase of average size of top coal. The dynamic evolution equations of the initial boundary of top coal on the support side and gob side are proposed based on granular medium mechanics and the correction coeffi- cients of the equations are determined according to the physical experimental data. The correction coefficient of the equation on the support side (kR ) is in the range of 0． 30-0． 35 while the correction coefficient of the equation on the gob side ( | kL | ) is between 0. 25 and 0. 28. The effect of size distribution on the correction coefficients is also dis- cussed. The value of kR and | kL | increases with the increase of average size of top coal.