The underactuated robotic hands have great adaptive ability in unstructured environments, especially for space irregular-shaped non-cooperative object grasping. Two kinds of underactuated robotic hands are presented in this paper, which have 12 degrees of freedom but using only 3 actuators. Although the shapes of target objects are unknown, they can be grasped very well by taking advantages of passive components with springs in robotic hands. The springs are located at different units in these robotic hands, which lead to different grasping methods when the hands start working on a task. They are analyzed by using Matlab and Adams software, then the best grasping method can be found by comparing the kinematic data and operability index.