The invention relates to a biosensor for organisms, in particular to an unmarked combined few-mode fiber evanescent field biosensor. The biosensor comprises a light source as well as a focusing lens group, a first bare fiber converter, a few-mode sensing fiber, a second bare fiber converter and a photoelectric signal collector which are sequentially connected, wherein the sensing section of the few-mode sensing fiber is arranged in a sample cell. The few-mode sensing fiber comprises a sensing section of a bare fiber core and transferring sections, wherein the diameter of the sensing section is in a micron dimension, and the transferring sections are arranged at both ends of the sensing section and respectively connected with both ends through tapered transition sections. The evanescent field excited by the few-mode sensing fiber with the tapered transition sections at both ends has high erergy, biological identification molecules on the fiber surface not only form a weak conduction condition but also strengthen the interaction between the evanescent field and a substance molecule to be detected, thus the sensitivity of the biosensor is greatly improved.