Subtracting a photon from a squeezed vacuum state is an efficient method for preparing an optical Schr?dinger cat state. It is shown that the fidelity of the Schr?dinger cat state based on photon subtraction depends on the purity of squeezed vacuum state. A two-mode squeezed state of light with a purity of 0.993 is experimentally produced by a non-degenerate optical parametric amplifier, where the pump beam oscillates partially in the cavity. Especially, two-mode squeezed states with high purity can be obtained by the non-degenerate optical parametric amplifier in a wide range of pump power due to the presence of thermal effect.