A method for determining the coal injection quantity of a coal injection system of a blast furnace includes the steps of firstly, choosing a certain time after a coal injection process starts to serve as a starting timing point T1 and recording the weight W1 of a coal injecting tank at the timing point T1; secondly, taking the time when the injection process is completed as a second timing point T2 and recording the weight W2 of the coal injecting tank at the timing point T2; thirdly, computing the coal injection quantity per hour by subtracting the weight W2 of the coal injecting tank from the weight W1 of the coal injecting tank if the time interval between the timing point T1 and the timing point T2 is at least one hour; and fourthly, changing a new coal injecting tank for continuous injection if the time interval between the timing point T1 and the timing point T2 is less than one hour, recording the weight W0 when the new tank starts injection, stopping injection of the new coal injecting tank at a third timing point T3 when the time interval between the starting timing point T1 and the third timing point T3 is one hour, and recording the weight W3 of the new coal injecting tank at the third timing point T3. The initial weight of the new coal injecting tank is W0; deltat1=T2-T1, and the coal injection quantity within the deltat1 is equal to W1-W2; deltat2=T3-T2, and the coal injection quantity within the deltat2 is equal to W0-W3; and deltat1 + deltat2=1h, and the coal injection quantity per hour is equal to (W1-W2) + (W0-W3). The method for determining the coal injection quantity of the coal injection system of the blast furnace is capable of decreasing fluctuation of the coal injection quantity and creating conditions for stable and smooth running of the blast furnace.