Abstract: Aimed at the problems of support parameter around the longwall top-coal caving mining in steeply thick coal seam,it determines the importance of the side plate’ s anti-extrusion capacity based on analyzing the relationship of support and surrounding rock in the working face. The force of resisting dumping and glide on the side plate of sup- ports is calculated by combining the method of drawing downward segment by segment while drawing upward in each segment with supports’ mechanical equilibrium equation. The side plate’s anti-extrusion capacity eventually presents 763 kN which is obtained from distinct element numerical calculations established in PFC2D based on the extremely in- clined panel No. 1201 in Dayuan Coal Mine,then the function of support is proved credible in fully mechanized caving face equipped with ZFY4800 / 17 / 28. Analyzing influences of support stability,the measures,such as equipping support with large aspect ratio,low center of gravity and low weight,matching general mining height,adjusting face inclination and increasing frictions among support,roof and floor,are put forward to decrease the unfavorable factors for supports and ensure fine guide and adjust function.