High efficiency CsI∶Tl crystal is used as X-ray converter in the detection system. The response characteristic of CsI∶Tl crystal to X-ray is very important in precision image processing. It is difficult to obtain a stabile H-D curve of X-ray film because of some uncontrollable factors in its processing. The response of CsI∶Tl crystal to X-ray has been theoretically proved to be linear. Some experiments on the 60Co X-ray source are performed to prove the linear of the response and the quantificational H-D curves of the CsI∶Tl crystal with different width are obtained. These H-D curves definitely show that the response is linear and that the linearity is very good. The linearity of less 0.15% is obtained from the experiment. This solves some unconquerable problems such as obtaining a stable, repetitive H-D curve which have existed for a long time in a X-ray film system.