Abstract: When supplied with radio-frequency power, quadrupole rodset with four cylindrical electrodes mainly produces quadrupole electric field. Quadrupole field can be used for the ion mass analysis, ion storage and ion transmission etc. Theoretical analysis shows that the pure quadrupole field can only be produced by four hyperbolic cross sectional electrodes. In other cases, higher order fields will be produced, such as hexapole, octopole, decapole, dodecapole and so on. Electric field distribution and the performance of the quadrupole rod system which were composed of four cylindrical rods with different diameters were investigated with numerical calculation. The results show that some higher order fields, such as A₃, A₄, A₅, A₆, A₁₀ can be added to the quadrupole field when the quadrupole rod system is composed of three identical rods and one bigger rod, and the quantity of each field can be adjusted by changing the diameters of the rods, or rotating the bigger rod with some angle. The quadrupole rod system with added higher order fields can still has good mass resolution. And it could also have higher ion transmission efficiency and better tandem mass spectrometry analysis ability, the quadrupole rod system given in this paper will have potential practical value.