Exploration of unknown environments using autonomous robots has been considered as a fundamental problem in robotics applications such as search and rescue [11], industrial inspection and 3D modelling. For exploration, the basic requirement for robots is to scan unknown space or detect free space as fast as possible. UGV (unmanned ground vehicle) [13] and UAV (unmanned aerial vehicle) [2] both have been employed for such a task with differences primarily in: (1) UGVs are more payload-capable. A ground vehicle can carry heavy, long-range laser scanners which are inapplicable for weight-constrained UAVs; (2) UAVs have superiors mobility and agility. UAV can fly above obstacles and cover areas that are inaccessible to UGVs, like obstacle's top surfaces. Consequently, a UGV often enjoys a larger sensor-coverage, yet cluttered and view-blocking environments could hamper its performance; on the other hand, a UAV may deliver inferior exploration efficiency due to its short-range sensor, but enjoys unblocked downward-looking view. Therefore, UGV favors open areas while UAV prefers cluttered places.In this paper, considering the environmental preferences, we propose an autonomous collaborative framework which utilizes their complimentary characteristics to achieve higher efficiency and robustness in exploration applications.For robotics exploration, [13] first proposed the concept of frontier, which is defined as unknown grid-map cells adjacent to free ones and thus represents accessible new information. Harmonic function, the solution to Laplace sEquation, is used to plan path to frontiers [5]. This method generates a scalar field in free-space based on its surrounding boundary conditions (occupied cells and frontier cells) and obtains the path using gradient-descent. For air-ground exploration, [1] uses the UAV as an back-up instead of an independent explorer. It is only deployed when UGV encounters high, invisible areas.[4] is also proposed based on the same spirit that one vehicle helps another, failing to exploit both vehicles' full potential. Compared to these works, the collaborative system proposed in this paper fully utilizes advantages of different vehicles and thus results in a more efficient exploration. We summarize our contribution as: 1. An efficient exploration framework that combines UAV and UGV's advantages. 2. A more efficient computation method of harmonic function for robotic exploration tasks. 3. Integration of the proposed collaborative exploration framework with the state estimation, sensor fusion and trajectory optimization. Extensive field experiments are presented to validate the efficiency and robustness of the proposed method.