An adjustable protective guard is formed by grids and a multi-dimensional connecting piece in a connected mode. The multi-dimensional connecting piece comprises a one-dimensional straight joint pipe, a second-dimensional right-angled joint pipe, a three-dimensional right-angled joint pipe and a three-dimensional extension joint pipe. The one-dimensional straight joint pipe is formed in a manner that two connecting pipes are fixedly connected to an entity, and the two connecting pipes are placed on the same straight line; the second-dimensional right-angled joint pipe is formed in a manner that two connecting pipes are fixedly connected to an entity, and a right angle is formed between the two connecting pipes; the three-dimensional right-angled joint pipe is formed in a manner that three connecting pipes are fixedly connected to an entity, and a right angle is formed between every two connecting pipes; the three-dimensional extension joint pipe is formed by three connecting pipes, one ends of two connecting pipes are respectively connected to the middle of the outer surface of the other connecting piece, and a right angle is formed between every two connecting pipes s; external screw teeth are arranged at the two ends of each grid. The adjustable protective guard can be combined freely according to shapes and sizes of objects to be protected, installation is quick, easy and convenient, materials can be saved, and cost can be lowered.