Optical microlenses have important applications in optical imaging, signal detection, biosensing, and other fields. As existing solid microlenses have invariable focuses and are biologically incompatible, chloroplasts in cells are used as natural microlenses, and the focusing properties of chloroplast microlenses and the application of such microlens in optical imaging and signal detection are studied. The results show that chloroplast microlens can focus incident lights with different wavelengths. The optical force generated by optical tweezers can be leveraged to control the shapes of the chloroplasts, and thereby adjust the focal length of the chloroplast microlens. The focal length can be adjusted in the range of 15--45 μm. Due to their ability to focus light, chloroplast microlenses can be applied to the imaging of subwavelength structures and the enhancement of fluorescence signals. In the experiment, optical imaging of the grating structure with a linewidth of 200 nm and actin filaments inside cells, as well as detection and enhancement of the fluorescence signal of quantum dots are achieved by the chloroplast microlens.