To study the calculation method of launch window in space fly-by problem, and the influence of different initial phase angles on it, a method is proposed. The general variable law to solve the Lambert problem is studied. On this basis, the process and timing sequence of space fly-by are analyzed. A method to calculate the launch window satisfying the constraints of fuel consumption and fly-by time is proposed with the influence of different initial phase angles. The length and region of launch window can be obtained through the method. Simulation results indicate that the method is effective and can provide valuable references for the orbital planning and design of space fly-by missions.