The Lee code is applied to characterize the plasma focus in two plasma focus devices UNU/ICTP PFF and Amirkabir plasma focus device (APF), and for optimizing the nitrogen soft x-ray yields based on bank, tubes and operating parameters. It is found that the soft x-ray yield increases with changing pressure until it reaches the maximum value for each plasma focus device, with keeping the bank parameters, operational voltage unchanged but systematically changing other parameters.