1. A natural language based service selection system for complementing incomplete queries, comprising: a semantic analyzing device which analyzes an incomplete query from a user semantically; a service selecting device which complements the incomplete query based on a semantically-analyzed query to complete the incomplete query, and selects a corresponding selected service from among a plurality of services based on a complete query; a retrieving device which retrieves an answer according to the corresponding selected service; and at least one processor coupled to the semantic analyzing device, the service selecting device and the retrieving device, wherein the service selecting device comprises a semi-automatic service selecting section which searches lost content in the incomplete query by using a service mapping rule base and complements the lost content through an interacting with the user, wherein the semi-automatic service selecting section comprises: a lost content searching unit which matches the semantically-analyzed query with a service mapping rule in the service mapping rule base, and extracts a service type to which the incomplete query belongs and a lost parameter in the incomplete query; a user interacting unit which prompts the user to input prompt information of a parameter value corresponding to the lost parameter and receives feedback information including the parameter value from the user; a parameter value extracting unit which extracts the parameter value from the feedback information of the user; and a query complementing unit which adds the service type, the lost parameter and the parameter value into the semantically-analyzed query to form the complete query.