Abstract: In the process of fully mechanized caving mining, the initial and periodic caving characteristics are different due to the influence of top-coal moving boundary. Taking the 12309 working face of Wangjialing coal mine as the background, the initial emission characteristics and rules of top coal in fully mechanized caving face are studied by combining theoretical analysis, field measurement and physical simulation test. The results show that: The equivalent diameter of the top coal was divided into four sections: 0~9cm, 9~18cm, 18~27cm, and 27~36cm. The mass fraction of the top coal in each section was 13.83%, 46.31%, 20.34%, and 27.36%, respectively；According to the similar simulation experiment, the correction coefficients of gravity acceleration on the left and right sides of the discharger are 1/108 and 1/92 respectively. After introducing the coefficient into the improved b-r model, it is calculated that the discharging time of 20309 working face is 33.95s, and the error with the measured discharging time is only 7.74%, which can accurately describe the discharging characteristics of top coal.