Abstract: Inclinometer is widely used in slope deep displacement monitoring, and the error of an inclinometer is an issue of concern. The existing research on the inclinometer error is based on the system error, and the error caused by the calculation principle of inclinometer monitoring data was seldom examined. Based on the consistency between the calculation principle of inclinometer monitoring data and the Euler solution of the ordinary differential equation in numerical analysis, this paper discusses the monitoring error of inclinometer according to the principle of numerical analysis method. The analysis results show that the monitoring error of an inclinometer is related to the initial value of the inclinometer, the length of the measurement section and the shape of the displacement deformation curve. Among them, the change of the initial value of inclinometer monitoring will bring errors to the monitoring. The length of the measuring section of the inclinometer has a linear relationship with the monitoring errors, and the errors decrease with the decrease of the measuring section length. When the displacement deformation curve is straight, the errors are zero; when the displacement deformation curve is a single convex curve or concave curve, the monitoring errors increase with the increase of the curve deformation degree and the orifice deformation displacement. The error of displacement deformation monitoring is the cumulative error, and the error of orifice is larger than that of the bottom. When using an inclinometer for monitoring, in order to reduce the monitoring error of the inclinometer, the initial value of each monitoring should be fixed, and the length of the measuring section should be reduced as much as possible.