With the vigorous development of small satellite technology, there is an increasing desire to use multiple, highly autonomous, earth bservation small satellites working together to complete more complicated earth observation missions through autonomous collaboration. Distributed Satellites System (DSS) composed by several small satellites is envisioned in the near future because of its capability of higher performance, lower cost, better fault tolerance, reconfigurability and upgradability. Considering the characteristics of autonomous operation cooperatively for DSS in future, the task allocation becomes a very important process in the operation. In the paper, with the principle of lowest general energy-consumption for completing observation task, a model for Constraint Satisfaction Problem (CSP) on task allocation was put forward by analyzing the features of observation task for DSS. Based on it, citing contract net protocol in MAS theory, an algorithm was presented to solve the model and an example of the earth observation satellites was given to validate the model and algorithm at last.