Aim To observe the expression changes of intermediate conductance calcium-activated potassium channel (KCa3.1) and large conductance calcium-activated potassium channel (KCa1.1) in rat thoracic aorta smooth muscle cells induced by high phosphorus in alkaline environment; To explore the relationship between the KCa and the phenotype transformation of rat thoracic aorta smooth muscle cells. Methods Tissue block adherence method was used to culture primary rat aortic smooth muscle cells. Vascular smooth muscle cell (VSMC) calcification model was prepared with 10 mmol/L β-glycerol sodium phosphate. PH value of culture medium was regulated by using HCl and NaHCO₃. Then the cells were divided into 5 groups:normal pH 7.4 group, high phosphorus pH 7.4 group, high phosphorus pH 7.7 group, high phosphorus pH 8.0 group, TRAM-34 intervention group, and cultured for 4 days. The expressions of KCa3.1, KCa1.1α, KCa1.1β, runt-related transcription factor 2 (Runx2) and smooth muscle 22 α (SM22α) were detected by reverse transcription polymerase chain reaction (RT-PCR) in each group cells. Results Compared with normal pH 7.4 group, the expression of Runx2 was increased in high phosphorus groups, and increased with the increase of pH (P<0.05); the expression of SM22α was reduced in high phosphorus groups, and reduced with the increase of pH (P<0.05). Compared with normal pH 7.4 group, the expression of KCa3.1 was increased and the expression of KCa1.1α was reduced in high phosphorus pH 7.4 group (P<0.05). In the high phosphorus groups, the expressions of KCa3.1 and KCa1.1α were increased with the increase of pH (P<0.05). In the same group, the expression of KCa3.1 was more than KCa1.1α (P<0.05). There was no significant difference in KCa1.1β expression among 3 high phosphorus groups (P>0.05). Compared with high phosphorus pH 8.0 group, the expression of Runx2 was decreased and the expression of SM22α added in TRAM-34 intervention group (P<0.05). Correlation analysis showed that KCa3.1 expression was positively correlated with Runx2 expression (r＝0.945, P<0.01) and was negatively correlated with SM22α expression (r＝－0.926, P<0.01). KCa1.1α expression was negatively correlated with Runx2 expression (r＝－0.746, P＝0.029) and was positively correlated with SM22α expression (r＝0.971, P＝0.002) in normal pH 7.4 group and high phosphorus pH 7.4 group. KCa1.1α expression was positively correlated with Runx2 expression (r＝0.805, P＝0.002) and was negatively correlated with SM22α expression (r＝－0.806, P＝0.005) in high phosphorus pH 7.7 group and high phosphorus pH 8.0 group. The expression of KCa1.1β was not correlated with the expression of Runx2 and SM22α (r＝0.414, P＝0.356; r＝－0.155, P＝0.714). Conclusion The expression of calcium-activated potassium channel in smooth muscle cells is involved in the phenotypic transformation of rat thoracic aorta smooth muscle cells induced by high phosphorus in alkaline environment.