Four Zn0.97Cr0.03O films were deposited on quartz wafers in various oxygen environment (0, 0.05, 0.15 and 0.2 Pa) using pulsed laser deposition (PLD). The films were characterized by XRD, PL, XPS, magnetic and electrical properties. Experimental results indicate that: (1) All the films are well crystallized and display a pure orientation. (2) All the films have ferromagnetism, and the film deposited at 0.15 Pa has the biggest Ms. (3) There exist VZn, Oi, Zni, VZn- and VO defects in the four films above, and the percentage of resonance peak area for VZn to the total area of all defects as a function of oxygen pressure is similar to Ms, which means that the magnetizations of the samples are closely related to Zn vacancy VZn. There is a Cr3+ state in the four films when the content of Cr3+ is the largest at 0.15 Pa. To sum up, the experimental results indicate that the substitutive Cr in the oxidation state of t3 and the neutral Zn vacancy in the Zn0.97Cr0.03O films is the most favorable defect complex to maintain a high stability of ferromagnetic order, which is consistent with the calculated results by the first-principle calculations.