In this paper, the influences of fiber core structure on photonic band gap and transmission loss for hollow-core photonic crystal fiber are investigated, and the proper fiber core structure of fiber preparation technology is obtained. First, the band gap structure of triangular lattice of hollow core photonic crystal fiber with a fixed duty ratio is calculated by using plane wave expansion method. When the transmission wavelength λ=1.55 μm, the structural parameters of the optical fiber are figured out. The value range of the core diameter is given by simulating the influences of core diameter on the band gap location and size, and the value of core wall thickness is obtained through analyzing the leak loss characteristics. Then the fiber end view drawing is designed according to the analytical results. The mode field distributions are simulated by the full-vectorial finite element method under different core diameters. Through the contrast analysis the best fiber core radius with R=1.6 Λ—1.75 Λ is obtained. The results indicate that choosing appropriate core structure not only can meet the photonic band gap and loss characteristics of hollow-core photonic crystal fiber, but also can properly reduce the difficulty in the preparation technology of fiber.