The Co-Al-CO32- layered double hydroxide（LDH） anti-corrosion film was formed in-situ on the surface of steel by hydrothermal method to improve the corrosion resistance of building steel. Effect of urea， sodium carbonate and sodium bicarbonate as alkali sources on the reaction system was studied to reveal the key factors of growing LDH film on steel surface. In addition， the microstructures of LDH films grown under different urea concentrations were compared and the corrosion resistance of LDH films was characterized by electrochemical method to analyze the effect of urea concentration. The results show that LDH can be in-situ grown on steel surface only when urea is used as the alkali source. Urea is neutral at room temperature， and Al³⁺ is hydrolyzed to form amorphous aluminum hydroxides which are deposited on steel surface as LDH crystal nuclei. The hydrolysis of urea accelerates when the reaction temperature increases. Therefore， the alkaline environment of the solution causes LDH nuclei to grow into films. What's more， the LDH sheets prepared under low urea concentration have more pores and larger size of LDH sheets； while the LDH sheets prepared under high urea concentration have smaller size and will squeeze each other to crack. The LDH film prepared under urea concentration of 0.375 mol/L grows compactly and has satisfied corrosion resistance.