For rotary drilling bored grouting pile construction in thick clay, silt and sand layer, borehole collapsing, drill rod burying, drill rod sticking, diameter shrinking and pile breaking often occur because of technical defects such as mud surpplying at borehole orifice and non-recirculation. On the premise of analysis on the mechanism in which low viscosity, fluid loss reduction and speed-up cuttings precipitation were solved, the formula of FCLS LV-CMC mud with low viscosity, high-level fluid loss reduction and anti-precipitation was put forward with simultaneous mud surpplying at hole bottom; hole cleaning by circulating mud was unnecessary to improve working efficiency.