Based upon the dispersion principle of cylindrically bent crystal, a transmissioin crystal spectrometer has been presented primarily to characterize the hotelectron energy distribution of laser generated plasma by registering the hard Xray Bremsstrahlung spectrum emitted by hot plasma in a larger energy range. Actually according to the different experimental requirements and variety of the distance between source and crystal, Johann adjustable Xray transmission spectrometer is designed. The focal slit of the spectrometer is planned to be movable, and its place can be changed when the distance between the source and crystal is changed. By the theoretical calculation the relation is acquired about the position of the focal slit and the distance from source to crystal, and the consequence can also be reached as long as Bragg angle is less than 0.2 rad that the incident light will approximately focus on one point. Furthermore, the primary theoretical analysis and conclusion on the questions of designing the device are illustrated.In conclusion, the adjustable focal slit breaks the limit of fixed distance from source to crystal; moreover, the frontal replaceable crystal makes the spectrometer more flexible.