Aiming at the problems of insufficient bearing capacity and weak scour resistance of the traditional suction caisson (TSC) for offshore wind power, a new type of modified suction caisson (MSC) is proposed. The MSC adds an external structure (ES) in comparison with the TSC. In order to clarify the seepage behavior in sand of the MSC for suction-assisted installation, the hydraulic gradients (HGs) are analyzed by numerical simulation when the suction is generated only in the internal caisson (IC) and when the suction is generated in both the IC and ES. The results show that the seepage behavior of the MSC is similar to the TSC when the suction is generated only within the IC. When the suction is generated both within the IC and ES, the trend of the HG outside the IC shows an “S”-shaped curve. Moreover, the “0-seepage line” is found, which causes the effective stress of the soil around the outer sidewall is decreased. The variation trends of the HGs of soil inside and outside the ES have a similar trend in comparison with the TSC. This study is helpful in predicting the resistance of the MSC during installation.