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Based on the research of SLA, SNAP, MetaService and contractual computing service, a service level QoS mechanism and algorithm for astronomy data distribution and backup in grid environment is proposed. A weight based comparison method for different data services provided by data servers is described first. The method can choose suitable data servers under the requirements of consumers. Based on the weight based comparison method, a data distribution algorithm is designed and this algorithm can obtain an optimized data allocating solution with a time complexity of O(x lg x + nxy) . In a practical example of data distribution and backup, the balancing mechanism is explained between two factors: the performance and the cost. The mechanism and the algorithm have been implemented in an astronomy data management prototype system. Compared to non- QoS guarantee scenario, a set of experiments with different weights of performance and cost have been tested and the results show that the QoS guarantee method can satisfy consumer's requirement effectively.