In order to overcome the problem that the image contrast and collection efficiency of on-site fingerprint images are usually quite low due to complicated background color at the crime scene, a portable fingerprint image acquisition system with a smartphone was proposed. In the system, a microprocessor connected to a smartphone through bluetooth can electrically switch light sources of different wavelengths. Multi-spectrum images were acquired to improve the fingerprint image contrast. With the help of the microprocessor, the smartphone can automatically capture fingerprint images under the illumination of different light sources. To ensure that the image quality can meet the requirements of identification process, the images were further enhanced by using multi-spectrum fusion and orientation filtering. Results show that the proposed system can effectively collect, fuse, and enhance multi-spectrum fingerprint images. It takes less than 1s to acquire and process the images when the number of light sources is 4, which satisfies the requirement of on-site image acquisition. As the system has the advantages of low cost, automation, and high portability, it is expected to be widely applied in criminal investigation.