There are many error sources in wellbore trajectory measurement and it is difficult to correct them. In order to improve the precision of wellbore trajectory measurement, it is necessary to identify the main error sources. The error sources considered in the current wellbore trajectory measurement error calculation model are introduced. The model for calculating the contribution rate of each error source to the eigenvalue of the covariance matrix, the semi-axis length of the error ellipsoid, the volume or surface area of the error ellipsoid is discussed, and it is recommended to calculate the contribution rate of an error source to the volume of the error ellipsoid to analyze the contribution rate of the error source to the wellbore trajectory measurement error. And, based on the error contribution rate of each error source to wellbore trajectory measurement, the method for identifying the main error sources in wellbore trajectory measurement is proposed. The calculation example shows that the main error sources of the vertical section are the misalignment error of the tool X / Y axis, the depth random reference error and the depth scale error, and the main error sources of the inclined section are the misalignment errors, magnetic interference errors and the constant declination error, the depth scale error and the depth stretch error, the main error sources of horizontal section are the bottom hole assembly droop error, the magnetic interference error, the constant declination error and the depth stretch error. In horizontal well drilling engineering, the measurement error of wellbore trajectory can be effectively reduced by reducing the magnetic interference errors, constant declination error, depth errors and misalignment errors according to the main error sources.