The characteristic and the formation and loss mechanism of different particles of hollow cathode discharge in oxygen at 266 Pa are investigated by using fluid model. The model contains 11 kinds of particles and 45 reactions. Under this simulation condition, the negative glow regions corresponding to the surrounding cathodes overlap. The results show that there is a strong hollow cathode effect. The density distributions of different charged and active particles are calculated. The charged particle density is mainly located in the central region of the discharge cell. Electrons and O- are the main negative charges in the discharge system, and their density peaks are 5.0×1011 cm-3 and 1.5×1011 cm-3, respectively. O+ 2 is the main positive charge in the discharge system with a peak density of 6.5×1011 cm-3. Abundant active oxygen particles exist in the discharge system, and their density is much higher than that of charged particles. According to the density of active particles, the order is O, O2(a1Δg), O(1D) and O3.Furthermore, the generation and consumption mechanism of electrons, O- and O+ 2 are calculated in detail, and the generation and consumption paths of different active oxygen particles are also given. The results show that there is a complex coupling process among these particles. Each reaction generates a certain particle and consumes other particles at the same time, resulting in a dynamic equilibrium for these particles.