For tight oil reservoirs, because of ultra-low permeability, the radial flow is difficult to be obtained in pressure build-up testing which results in multi-solution of flow model and interpretation results. How to determine the right model and identify the right interpretation result are the key questions. This paper presents a well test interpretation methodology based on time and space scales to increase accuracy of well-test interpretation results. Two steps are performed to determine reasonable flow model and interpretation results based on time and space scales. For the time scales, the history of well testing data for a single well are interpreted and the interpretation results are compared first. Then for space scales, the well test data in space for multi-wells are analyzed. At this stage, the interpretation results of adjacent wells are compared. The reservoir geological data and the production history are used as a reference to appraise the well test interpretation results. After these two steps, the reasonable interpretation model and interpretation results can be determined. Two cases selected from Changqing oilfields whose permeabilities are almost under 0.5mD and fluid flow rates are very low are analyzed and the great application effect is attained. This methodology is applied in Changqing oilfields extensively and is proved to be a great methodology in ultra-low permeability oil reservoirs. It can be expanded to other unconventional reservoirs, such as tight gas, shale gas and so on. The innovation is that this paper derives a well testing analysis methodology to determine right model and identify the right interpretation result based on time and space scales. It can decrease the errors which are caused by multi-solution of flow model and many interpretation results.