In this paper a low-cost linear Paul trap demonstration device is introduced. Using a neon lamp transformer as high voltage power supply and four stainless steel cylinders as electrodes, an alternating quadrupole electric field can be obtained. When charged sporangium powder is sprayed into the region surrounded by the electrodes, sporangium with suitable charge can be captured by alternating electric field. Illuminated by a laser beam, localized trajectories of the captured particles can be clearly observed. The electric potential distribution of Paul trap is numerically calculated by using a simplified electric potential model in Matlab, and the particle trajectory is simulated by analyzing the gravity, electric field force and air resistance of the particle. The charge of the particles is also estimated.