Front-side releasing suspended structures by bulk micromachining process have the advantages of simplifying vacuum or hermetic packaging process and reducing squeezes-film damping. This paper study the effect of doping concentration on the etching rate of (111) plane. The experimental results show that increasing the boron doping concentration (but less than the self-stop etching concentration) significantly increases the etching rate of (111) plane and the etching rate ratio of (111) and (100) planes. This phenomenon is used to speed up the undercutting and release <110> oriented clamped-clamped beams, which can be used as piezoresistive sensitive elements. The etching depth when microbeams are just released and the silicon wedges under single beams, double beams and triple beams disappear is deduced and verified by experiments. It has a certain guiding significance for designing piezoresistive sensors.