Abstract: The Z-pinch is the most powerful X-ray source which can be obtained in the laboratory and it can be applied for many research fields, such as high energy density physics, inertial controlled fusion and laboratory astrophysics. In material experiments the Zpinch Xray source is utilized to radiate the material for carrying out thermodynamic effect researches, which are important for the radiationhardened reinforcement design of the system. In these experiments the radiated Xray power density acted on the sample is one of the most important parameters concerned by the researchers. In this paper the radiation filed of a Zpinch soft Xray source was analyzed. The Zpinch Xray source was treated as a line source and a Lambert radiation source or called as cosine radiator respectively. Based on the fundamental optical rules the expression was deduced which illustrated the Xray power density radiated by the Zpinch source. For the line source model, it shows that the radiation power density in the far filed is inversely proportional to the square of the distance from the source, and the contour surface is the conventional sphere surface, while in the near filed the radiated power density is inversely proportional to the distance, and the contour surface is more like a cylindrical surface. For the cosine radiator model the radiated power density in the far filed is still proportional to the square of the distance from the source, and the contour surface is a modified sphere surface which is multiplied by the cosine value of the angle. In the near filed the cosine radiation source model gives the same distribution for the radiated power density as the line source model. Based on the analysis mentioned above the radiation power density on the sample at Qiangguang Ⅰ accelerator was investigated. In Qianguang Ⅰ experiments the sample was located 5 cm far from the central Zpinch source, which implies that the distance is comparable with the size of the Zpinch source. The analysis results show that the power density radiated of the assessed material is inversely proportional to the square of the distance from the material to the source. In another word the radiation power density of the sample is satisfied the far filed criterion providing that the sample is located further than 5 cm, so it is reasonable in the experiments on Qiangguang Ⅰ that the power density radiated of the sample can be obtained by measuring the power density at the point which is further than 5 cm.