Optical fiber inclinometer technology has been gradually used in various projects. However, the error analysis of this method is decisive for its subsequent development and application. In this study, through three tests, the monitoring length, layout plane, and boundary conditions that may lead to measurement errors are studied in detail and the contribution of different factors to measurement errors is explored. The research results reveal that the measurement errors caused by monitoring length and layout plane have an empirical function relationship with the monitoring length <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> and the angle θ between the layout plane and the inclined direction. Any changes in the boundary conditions will result in an overall deviation in the calculated deflection curve. After analyzing the characteristics of each error, a comprehensive error equation is proposed. This equation has guiding significance for error correction and technical improvement in optical fiber inclinometer.