An optical fiber assembly includes a prism, four first collimating portions, four second collimating portions, two third collimating portion, two light emitting elements, two light receiving elements, four optical fibers, and two light intensity detectors. The prism includes an incident surface, a first reflecting surface, a first emergent surface, a second reflecting surface, and a second emergent surface. The first collimating portions are positioned on the incident surface. The second collimating portions are positioned on the first emergent surface. The third collimating portions are positioned on the second emergent surface. Each light emitting element faces one of the first collimating portions for emitting light beams. Each light receiving element faces one of the first collimating portions. Each optical fiber faces one of the second collimating portions. Each light intensity detector faces one of the third collimating portions for detecting light intensity of a corresponding light emitting element.