A collimator for adjusting an X-ray beam includes: an up-and-down adjustment mechanism; a left-and-right adjustment mechanism, a supporting member; and an adjusting plate connected with the supporting member. The up-and-down adjustment mechanism comprises first rotating nuts, up-and-down moving leading screws threadedly connected with the first rotating nuts, and upper and lower sliding stops located in the supporting member. Each of the leading screws is connected with the upper and lower sliding stops to drive the upper and lower sliding stops, respectively, to vertically move. The left-and-right adjustment mechanism comprises second rotating nuts, horizontally moving leading screws connected with second rotating nuts, and left and right sliding stops located in the supporting member. Each of the horizontally moving leading screws are connected with the left and right sliding stops to drive the left and right sliding stops, respectively, to horizontally move.