The interferometry with an x-ray laser as probe is an important tool of diagnosing the electron density of a high temperature and dense plasma. A successful experiment of diagnosing the density of a CH plasma was demonstrated by using a Ni-like Ag x-ray laser as a probe and a Mach-Zehnder interferometer as instrument under the ShenguangII laser facility. The legible interferogram indicating the information of electron density was obtained.