Quantum computing is a frontier research field in high performance computing, and photon system is one of the important ways to implement quantum computing. In optical quantum computing, the entangled photon source is used to generate photons to encode quantum information, and its performance affects the quality and quantity of quantum bits directly. This paper systematically studies the entangled photon source based on spontaneous parametric down conversion (SPDC), which is the most popular technique to this aim, analyzes the performance model to study purity and brightness of entangled photons generated by type-I SPDC, and derives the key condition to obtain multi-photon entanglement via two-photon entanglement. Meanwhile, using barium metaborate, this paper realizes two photons entangled based on type-I SPDC, and mainly verifies our performance model. The model and the experiment results provide theoretical and technical support for generating more quantum bits in optical quantum computing system.