The relativistic electron beam is used to excite argon and the effects of purity and pressure on 309 nm spectrum are studied. The experimental results of purity show that the intensity of 309 nm spectrum decreases with the increase of purity. These results are inconsistent with that the 309 nm spectrum would originate from the transition of H2O. When the pressure of argon is changed, there is a maximum of intensity at 0.08 MPa, and the intensity is almost changeless between 0.1 MPa and 0.2 MPa Thus, the 309 nm spectrum does not originate from the transition of excimer or ionic excimer, which is mainly formed by three body collision.