Abstract: In response to the problem of excessive length of blocked sections in the blasting of the open-pit mining platform of Dabaoshan，which leads to insufficient energy release at the top and high rate of large blocks，a field study on top layer detonation charge was conducted. A single-hole mechanism iden⁃ tification was carried out on the expansion mining platform 661，and a 75 holes group comparison was con⁃ ducted on the east slope platform 637. A layer detonation charge design was carried out and the top blocked length (5.5，4.0，3.5 m) was compared. Under the condition of maintaining the total charge amount basically unchanged and micro-differential initiation of each hole，the effect was evaluated by block size statistics. The results show that in the single-hole test，D50 decreased from 616 mm to 485 mm， D80 decreased from 947 mm to 834 mm，Dmax decreased from 2 074 mm to 1 443 mm，and the rate of large blocks greater than 1 m decreased from 15.2% to 6.1%. In the group comparison，the layer detonation charge + top blocked length of 3.5 m achieved the optimal and stable crushing effect，and the rate of large blocks decreased by 72% compared with the continuous detonation charge structure，while the blasting ef⁃ fect of the scheme with top blocked length of 4.0 m did not improve significantly. From the experiments，it can be known that without increasing the charge amount，top layered detonation charge can achieve energy and time-space redistribution，significantly improving surface crushing and pile shape. The blocked length is the key control quantity. Under the given rock properties and hole network conditions，3.5 m is the pre⁃ferred value. An excessively long blocked length is likely to cause insufficient energy release at the top，re⁃ sulting in incomplete top crushing.

Key words: blasting of the open-pit mining platform, deep-hole blasting, charging structure, layer detonation charge