Bridge influence lines contain rich structural information and are widely used in intelligent warning, damage identification, and state assessment of bridges. Therefore, accurate identification of influence lines from field test data is of utmost importance. This paper establishes a method to solve the influence line based on the basis function extension method, simplifying the high-dimensional problem of identifying the influence line to the low-dimensional problem of identifying the weight coefficient, thus fundamentally ensuring the stability of the result. Results indicate that, compared with traditional methods, the basis function extension method (BFE method) can effectively improve accuracy, reducing the mean absolute error (MAE) and root mean square error (RMSE) to 1.3E-3 and 4.8E-4, respectively, meeting engineering requirements.