The principle of the T-shaped exit fixed mechanism of the exit beam and entrance one in the dou- ble-crystal monochromator is introduced

and requirements of deciding the ubiety and dimensions of two crystals are analyzed. According to these requirements

a method of deciding the ubiety and dimensions of two crystals based on the T-shaped exit fixed mechanism is put forward. The method can not only satisfy the working requirements of dou- ble-crystal monochromator

but also optimize the length of two crystals

moreover

it can eliminate the position error when fine adjusting the second crystal. The advantage of the method is testified by giving an example in the end.