Abstract: Based on the conservation of momentum and energy, the derivation of Euler equation and Bernoulli equation was described in detail. From the derivation of the two equations, it can be clearly seen that the necessary and sufficient condition for Euler equation is that the divergence of the viscosivity stress tensor is zero, and that the fluid viscosity is zero is only a sufficient condition but not a necessary condition for Euler equation to be established. In addition to requiring Euler equation to apply, Bernoulli equation still needs to satisfy the conditions such that the fluid is barotropic, the mass force is potential, and the flow is steady. If the total energy of the system needs to be preserved, the condition of barotropic fluid should be changed to that of isentropic flow.