Newtonian mechanics assumes the essence of light is particle, Electrodynamics regards the essence of light is wave, while Quantum mechanics thinks the light is something that has the nature of wave-particle duality. According to Gödel incompleteness theorem, these theories could not be complete even if they were self-consistent. In fact, modern physics has not been able to give a unified explanation of the fundamental interactions in nature. In this article, a general field theory is developed based on an assumption that photon is an entity particle. In contrast with point particle, entity particle is an object that has both mass and volume. Photon is entity particle and is ubiquitous in the universe. Starting from the particle distribution in 3-dimensional Euclidean space, a set of partial differential equations for particle field is derived with the tools of vector analysis and field theory. This article analyzes the property of particle field, and explains the relationship of different fields and interactions. Particle field can be divided into three categories: density field, potential field and action field. Density field includes mass density and momentum density, which satisfy the laws of conservation. Potential field includes mass potential and momentum potential, which are the spatially correlated statistics of density field. Action field includes gradient field, divergence field and curl field, which are the first-order space change rate of the potential field. It is shown that the mass potential is equivalent to the gravitational potential and the electric potential. The momentum potential is equivalent to the magnetic potential. Gravitational field and electric field belong to gradient field, motional dissipation belongs to divergence field, and magnetic field belongs to curl field. The gradient of divergence field is the wave equation whose solution contains the light wave and the gravitational wave. The results clearly show that field is a statistical effect of a huge number of particles. Entity photon is the medium or ether that transmits light wave and gravitational wave, and the mass of photons is the so-called dark matter in the universe.