The invention relates to a determining method and device for the dosage of acid liquid for open-hole horizontal well matrix acidizing and belongs to the technical field of oil-gas field development. The determining method comprises the steps that firstly, a reservoir geological model is built, and then the dosage of the acid liquid is calculated according to the built reservoir geological model and the acidizing radius; then, the shield amount of shielding layers to acid liquid leak-off is calculated according to the distance between a bore hole of a horizontal well and the upper covering shielding layer as well as the distance between the bore hole of the horizontal well and the lower covering shielding layer; and finally, the dosage of the acid liquid subtracts the shield amount of the shielding layers to acid liquid leak-off, and the obtained difference value serves as the actual dosage of the acid liquid of the horizontal well. On the basis of a conventional matrix acidizing acid liquid dosage calculating method, the shielding effects of the upper and lower shielding layers of a reservoir on the acid liquid in the acidizing construction process are considered comprehensively; and by utilizing an equivalent calculating method, the shielding amount of the shielding layers to the acid liquid is calculated, accordingly, the actual dosage of the acid liquid in acidizing construction is calculated, the unreasonable dosage of the acid liquid is decreased, and the construction cost is reduced.