The invention discloses a preparing process of low-cost formate drilling flushing fluid. The preparing process includes: 1, heating water to above 80 DEG C, adding sodium bentonite, and performing uniform mixing at the speed of 600 to 800 rpm; continuing mixing at the speed of 6000 to 8000 rpm for 30 to 40 min, and performing curing; 2, adding in order sodium formate, coating agent for drilling fluid, and inhibitor for drilling fluid, and performing mixing for 30 to 60 min; 3, adding in order well-wall protectant HF-GRA-II for drilling fluid, filtrate reducer for drilling fluid, blocking agent for drilling fluid, waterborne lubricant NSR-1 for drilling fluid, low-fluorescence sulfonated bitumen, and tackifier; and 4, adjusting pH to 8 to 9 with caustic soda, and adding weighting agent to adjust density to 1.15 to 1.25 g / cm<3>. The preparing process has the advantages such that anti-collapse ability is high, the cost is lower and the applicable range is wide.