Stokes and anti-Stokes competing processes of producing XUV coherent radiation by two-step excitation in He/K mixture vapour was discussed. The relative transition oscillator strengths, SRS gain coefficients and thresholds were calculated. The processes of producing 64.3 nm coherent radiation has the larger gain coefficient and less threshold compared with those competing processes. But the anti-Stokes processes to level K[3p54s22p1/2,3/2] and the stokes processes to levels K[3p53d(3P)4s2P1/2,3/2] will compet seriously with the producing process of 59.8 nm XUV coherent radiation.