The inorganic scale formed due to the incompatibility between injected water and formation water entangles with crude oil and particles in the pores and deposits，reducing the flowable space of the pore throats. The effect of single acidification is not ideal，restricting the water injection development of the oilfield. In order to improve the effect of the injection measures，it is necessary to clarify the characteristics of scale formation during water injection and the characteristics of scale dissolution during acidification. Taking the Chang X ultra-low permeability reservoir in Changqing oilfield as an example，through the visualization of the reservoir 's own fluid，external fluid and acidification times and the core simulation experiment scheme，combined with the changes of scale and injection pressure in the pore throat，the scale size，distribution change and sedimentary morphology characteristics at different positions were analyzed，and the changes of plug dissolution，migration and deposition during acidification were revealed. The results showed that in water injection development，due to the influence of water-type incompatibility，the inherent oil of the porous medium of the stratum，the migration and adsorption of particles，and the aggregation of secondary and tertiary precipitates during each acidification with the scale at the same location，the blockage gradually transited from a dotted composite scale composed of inorganic and organic matter to block and strip，moreover，the larger the proportion of heavy components in crude oil was，the more severe the scaling was，and the larger the size of the scale body and the scaling area were. The easily soluble scale due to acidification was mainly carbonate scale，which was concentrated in the pore throat channels，and its morphological distribution was layered，dotted and streamlined. Insoluble scale was a complex scale mixed with crude oil and inorganic substances，which was concentrated at the blind end，pore wall and inlet end. It was difficult for ethanol to dissolve it at the reservoir temperature；and petroleum ether，which was prone to dissolve heavy components，was difficult to enter the chip，increasing the difficulty of unblockage. Under the same simulated crude oil composition and the process of non-backflow of residual acid，the more times of acidification，the worse the unblocking effect. The unblocking area at the injection end was larger than that at the extraction end. The unblocking effect of the main flow channel was significantly better than that of the branch channel，and some dissolved and reduced microscopic particles migrated backward. When the well was re-trapped and water was driven to form scale，affected by the cumulative effect of in-situ scale formation，the closer to the inlet end，the more secondary blockages were generated，and at the same time，large holes transformed into medium and small holes. The comparison of different processes showed that the effect of residual acid reflux was much better than that of direct water injection after acidification.