Based on the theoretical study and field practices in magnetic ranging in well drilling， a model has been constructed for determination of the relative distance and direction between the downhole drill bit and target with the distance correction method and the Kalman filtering method for weak magnetic signals proposed； thus solving the problem with long distance and high precision magnetic ranging in CBM horizontal well drilling in the complex operation environment. The field test and application results showed that the calculation method for distance correction can obtain the value well approximate to the one measured in the interference-free environment； thus eliminating effectively the measurement error caused by downhole magnetic moment. The noise and clutter had a significant influence on the value of single measurement； however， through Kalman filtering， the measurement value was more accurate than the one from single positioning calculation with the measurement error reduced from 1.9m to 0.14m at the distance of 40.06m. In magnetic ranging for the “U” type CBM horizontal well intersection， the effective target can be controlled within 177.8mm of the vertical extraction well.