The toroidal field (TF) magnet of the Joint TEXT (J-TEXT) tokamak consists of 16 D-shaped copper coils interlinked in series. In this study, the finite element simulation software ANSYS is used to calculate the toroidal magnetic field and its ripple over the plasma region. Moreover, the possible error field caused by the busbar and the mounting error or noncoaxiality among the TF coils are also calculated. Results show that the busbar lead-in and the possible outward radial shift of a single TF coil will cause a relatively large error field.