The invention belongs to the technical field of sensors and provides a concentration sensor for controlling a single microsphere based on an optical fiber. The concentration sensor comprises a microfluidic chip, a mono-dispersed microsphere, a track sample feeding pump, a micro-sampler, a single mode optical fiber, a pump laser device and a microscope with a CCD. The concentration sensor is characterized in that the microfluidic chip is internally integrated with a microfluidic channel, one end of the single mode optical fiber is a flat end surface, the flat end surface is put into the microfluidic channel and is coaxial with the microfluidic channel, and the other end of the single mode optical fiber is connected with the pump laser device; the mono-dispersed microsphere is added into a to-be-tested solution, and the to-be-tested solution is extracted by virtue of the micro-sampler and is injected into the microfluidic channel by virtue of the track sample feeding pump; and pump laser generated by the pump laser device is transmitted by the single mode optical fiber and is emitted by the flat end surface, and the distance from the microsphere to the flat end surface of the single mode optical fiber is detected by virtue of the microscope with the CCD so as to realize concentration sensing. The concentration sensor is high in sensitivity and low in preparation cost, and is easily integrated with the microfluidic chip, and a machining process is simple.