The functional complex Mn-TCPP-CS20 as a potential magnetic resonance imaging (MRI) contrast agent was synthesized through tetra(4-carboxyphenyl) Mn(Ⅱ)-porphyrin (Mn-TCPP) modified by CS20, which was low degree of polymerization and narrow distribution. The results showed that Mn-TCPP-CS20 had good water-solubility and structural stability. The chemical structures of the products were characterized with Fourier transform infrared spectra (FTIR), UV-Vis spectra, mass spectrum (MS) and inductively coupled plasma atomic emission spectrometry (ICP-AES). The results showed that Mn-TCPP was successfully linked to CS20 by an amide function. The relaxation properties in vitro of the functional complex Mn-TCPP-CS20 as the potential MRI contrast agent were preliminarily studied. It was found that the longitudinal relaxivity (r1) of the synthesized Mn-TCPP-CS20 (6.11 mmol-1·L·s-1) was higher than that of the commercial contrast agent Gd-DTPA (r1=3.59 mmol-1·L·s-1). Besides, the imaging effect of Mn-TCPP-CS20 was superior to that of Gd-DTPA in the same condition. These studies suggested that Mn-TCPP-CS20 has the advantage of becoming a potential tissue-targeting contrast agent.