The invention provides a preparation method of an irregular easily-broken coal rock block test piece. The preparation method is suitable for manufacturing of rock mechanics test pieces of irregular easily-broken coal rock mass. Waterproof materials are coated on the surface of an easily-broken or random-shaped coal rock block, the coal rock block is arranged in a box mould suitable for the size of the coal rock block, a layer of releasing agent which is not reacted with cement is evenly coated on the inner surface of the box mould, and prepared cement grout is filled in the box mould. After the box mould is filled densely and arrives at design strength, a core drilling rig is used for drilling a column-shaped test piece, cement grout solidification sections at two ends of the column-shaped test piece obtained through drilling are cut off through a rock cutting machine, only a coal rock block section is remained, and a required coal rock test piece is obtained through planishing. The preparation method of the irregular easily-broken coal rock block test piece overcomes defects that the irregular block is not prone to clamp and broken coal rock is easy to damage, and the processed test piece guarantees mechanical properties of an original coal rock block to the maximum extent. The preparation method of the irregular easily-broken coal rock block test piece is simple, easy to operate in a machining process and low in cost and has strong practicability.