In this work, the distributions and releasing properties of the primary volatile products during co-pyrolysis of low-rank coal and green algae (GA) has been studied using fixed-bed reactor with online mass spectrometry. Surface morphology of the char was described quantitatively by SEM combined with fractal theory. Different forms of synergistic effects existed from both the yields of products and composition of the main gaseous products. Positive synergistic effects from tar yield were observed under 25% of GA from 600 to 850 degrees C, indicating GA promoted the formation of tar. Opposite synergistic effects on the content of H-2 and CO were gained when the mass ratio of GA was 50%. The fractal dimensions of co-pyrolysis char were less than the calculated values except under 650 degrees C, which meant the GA promoted the homogeneity of char surface. This work could provide essential data for proper operation parameters selecting for co-pyrolysis.