For the beam transport system in storage ring, linear coupling is mainly caused by skew quadrupole and longitudinal solenoid field.

Characterized by its compactness,convenience,high current and high transmission,radio frequency quadrupole(RFQ) accelerator is fit for accelerating the low-velocity ions directly from ion source and has become a common injector in low-energy end of main accelerators for many applications such as HPPA,SNS,ADS and RIB facilities.

The paper shows equipote ntial surface equation, field line equation, the graph of field lines and equipo tential lines of the linear quadrupole field and then proves the orthogonality b etween the field line and equiopotential surface of the linear guadrupole field.