1. A Layer 2 adapter selecting system, including: a Layer 3 packet requirements extracting module, a destination Layer 2 address acquiring module, a Layer 2 adapter metric acquiring module, an appropriate Layer 2 adapter identifier selecting module, a packet caching module and a packet scheduling module; wherein, the packet caching module is configured to cache a received Layer 3 packet; the Layer 3 packet requirements extracting module is configured to read the Layer 3 packet from the packet caching module, extract the type and parameter value of a requirements from the Layer 3 packet and transmit the obtained results to the appropriate Layer 2 adapter identifier selecting module; the Layer 2 adapter metric acquiring module is configured to acquire instant metric parameter values of each of the adapters and transmit the obtained results to the appropriate Layer 2 adapter identifier selecting module; the appropriate Layer 2 adapter identifier selecting module calculates an adapter identifier and a Layer 3 packet metric according to the type and parameter value of the requirements calculated by the Layer 3 packet requirements extracting module and each adapter metric value calculated by the Layer 2 adapter metric acquiring module, transmits the resulted adapter identifier and the Layer 3 packet metric to the packet scheduling module, determines the link corresponding to the adapter is a unidirectional link or a bidirectional link according to the obtained adapter identifier, and if the link is a unidirectional link, then performs parameter negotiation through a bidirectional link to obtain a unidirectional link destination address mapping parameter, and transmits the mapping parameter to an external destination Layer 2 adapter through the bidirectional link; the destination Layer 2 address acquiring module is configured to acquire a Layer 2 address of each of the adapters associated with a destination Layer 3 address; the packet scheduling module calls an external Layer 2 adapter driver according to the adapter identifier and Layer 3 packet metric obtained by the appropriate Layer 2 adapter identifier selecting module, wherein the Layer 2 adapter driver performs packaging and sending of the packet.