To clarify the physical essential, a perspective of force balance to dynamic soaring was put forward. Equilibrium equations were derived according to physical laws. The solution of the equations solved mathematically formed a curve. Simulations of the curve were made in the cases of an albatross and an unmanned air vehicle in different wind gradients for instances. The results of analysis and simulation indicate that the curve is the velocity set that satisfies the equilibrium equations; the curve consists of a climbing branch and a diving one; the climbing branch exists only if the gradient is sufficiently large, and it is critical to dynamic soaring.