The quatum statistical properties of two-photon radiation from two-level atomic system are investigated by using complete set in the nth subspace of generalized Jaynes-Cummings hamiltonian, and the general expression for field variance is obtained. The conditions for optimum squeezing as well as the optimum squeezed quantity are found out for an arbitrary initial state, and the time evolution of the optimum squeezed state is then analysed. As an example, the squeezing in an initial state with bare atomic system in thermal equilibrium is discussed, which demonstrates the dependence of squeezing on several parameters, such as, temperature, detuning, coupling strength. The conditions for squeezing are given analytically and numerically. Finally, the physical picture of the squeezing is pointed out.