In order to discuss the mechanical behaviour differences between the two-tower and three-tower suspension bridges, based on the Taizhou Yangtze River Bridge, three dimensional space finite element models of two-tower, three-tower with concrete mid-tower and with steel mid-tower were constructed. Through the comprehensive analysis, main conclusions are got as follows: due to the mid-tower lack of effective restraint from side cable, compared with two-tower suspension bridges, three-tower suspension bridges have lower total stiffness and natural frequency, with larger deflection-to-span ratio of main girder. So, some factors such as anti-slipping safety factor between the main cable and saddle, deflection-to-span ratio of main girder and force in mid-tower, which are not important in two-tower design, become controlling ones in three-tower suspension bridges. These factors are related to anti-pushing rigidity of mid-tower, but in contradictory demanding for the rigidity of mid-tower. After comprehensive analysis, steel tower with shape of upside-down ‘Y’ meets all demands, and then, was selected as appropriate structure for the mid-tower.