The invention discloses a low-cost manufacturing method of a flaw-detection-ensured Q345B stage steel plate. The steel plate comprises, by mass percent, 0.15 to 0.20% of C, 0.15 to 0.50% of Si, 0.50 to 0.90% of Mn, not larger than 0.035% of P, not larger than 0.025% of S, 0.020 to 0.060% of Ti, 0.015 to 0.035% of Als, and the balance Fe and inevitable impurities. During preparing, molten iron is subject to KR desulfurization treatment firstly, the molten iron obtained after desulfurization treatment is subject to slag removal, the molten iron is converted into a converter to be smelted, outputmolten steel is subject to continuous casting and rolling, finally, the rolled steel plate is subject to stacking for slow-cooling for 24 to 72 hours, and then sampling inspection is carried out; thechemical components and the production process of the prepared Q345B stage steel plate are simple, the production cost is low, the flaw detection property is good, and the method has the good application and popularization prospects.