The spatial accessibility between inland city and seaport plays a very important role in the process of industrial transformation and upgrading and reasonable layout of port and hinterland. Firstly, this paper began with the difference between seaport and inland-city about the property and the traffic flow on traffic network to define the meaning of inland spatial comprehensive accessibility of port. Then, the modified gravity model was used to measure the comprehensive accessibility level of the inland ports in the Northeast China, analyze the different spatial characteristics and explore finally the mechanism of its internal. The results showed that: ①The accessibility level of the Yingkou Port grew fast and surpassed the Dalian Port at last, the Jinzhou Port followed the Dalian Port, the Dandong Port caught up later, with the Panjin Port and the Huludao Port being slightly behind, but it was contrast with the “n” structure feature measured by the shortest travel time index and the weighted average travel time index. At the same time, the difference among the six ports was more significant in the hinterland of the overall improvement of the traffic network. ②The inland spatial accessibility of the six major ports is presented along the Harbin Dalian traffic. The level of spatial accessibility in western Liaoning and eastern Mongolia is higher than other areas. There exists a kind of “enclave” phenomenon about the port-inland spatial accessibility of the capital city and the resource type city which have strong comprehensive economic strength. ③Traffic cost between port and inland resistance promotes the formation of the basic pattern of inland space in the port, and the important traffic lines and the comprehensive “quality” promote the change of this basic pattern.