The Co-Al-CO3-LDH anti-corrosion film was grown 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 with different urea concentrations were compared and the corrosion resistance of LDH films were characterized by electrochemical method to analyze the effect of urea concentration. The results show that LDH can in-situ grown on the steel surface only when urea is used as the alkali source. Urea is neutral at room temperature, and Al ions are hydrolyzed to form amorphous aluminum hydroxides which are deposited on the surface of steel 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 film prepared with low urea concentration has pores and larger size of LDH sheets; while the LDH sheets prepared with high urea concentration have small size and will squeeze each other to crack. Hence, the LDH film prepared at urea concentration of 0.375 M grows compactly and has satisfied corrosion resistance.