This study aims to address poor adaptability and timeliness in the design of smooth blasting hole parameters in the past. With the drilling and blasting method-based construction of large cross-section tunnels in Botanggou and Tielugou along the Zunhua?Qinhuangdao Expressway as the research background and Ⅲ and Ⅳ grade surrounding rocks of porphyritic granite as the research object, statistical regression analysis and theoretical derivation were conducted on the measured data during on-site drilling according to the rock solidity coefficient. Based on this, a simple parameter design method for a smooth blasting layer based on the drilling speed index of an air leg anchor rod drill was proposed, and the relationship between the drilling speed and the parameters of the smooth blasting layer hole was obtained. The research results indicate that there is a good correlation between the drilling speed and parameters such as the spacing between surrounding holes and the thickness of the smooth blasting layer. Based on the engineering conditions of the Botanggou and Tielugou tunnels, a parameter design table was calculated for the drilling speed of the drill rod, the spacing between surrounding holes, and the thickness of the smooth blasting layer. Through on-site practice, this method achieves good smooth blasting results, with a hole trace rate of over 90%. The research findings can provide a theoretical basis and reference for the design of smooth blasting parameters of tunnels in similar geological conditions.