The objective of this study was to evaluate the effects of feeding different levels of calcium (Ca) and phosphorus (P) diets to Yili mares during lactation period on milk composition and fatty acid composition of milk fat, and growth development and plasma physiological-biochemical indices of foals, in order to provide references for further understand nutrition of Ca and P for Yili mares. Twenty-five Yili mares in their end of second lactation month with similar age, body weight (BW) and parity were selected, and divided into five groups randomly according to the BW of mares and foals, each group with 5 mares. Dietary Ca and P levels of the five groups were 45.0 and 30.0 g/d (group Ⅰ), 48.5 and 32.0 g/d (group Ⅱ), 52.0 and 34.0 g/d (group Ⅲ), 55.5 and 36.0 g/d (group Ⅳ), 59.0 and 38.0 g/d (group Ⅴ), respectively. The trial lasted for 90 days, and every 30 days for a test cycle. The results showed as follows:dietary Ca and P levels significantly affected the average milking volume each time and estimated daily milk yield (P<0.05), but there were no significant differences among groups for milk fat percentage, milk protein percentage, lactose percentage and milk total solid content (P>0.05). Compared with groups Ⅰ and Ⅳ, average milking volume each time of group Ⅲ was significantly increased by 20.65% (P<0.05) and 15.22% (P<0.05). Subsequently, milk Ca content of group Ⅲ was 21.54% lower than that of group Ⅰ (P<0.05). Meanwhile, the contents of some fatty acids, such as myristoleic acid (C14:1), palmitoleate (C16:1), linoleate (C18:2n6c), arachidate (C20:0), cis-11-eicosenoate (C20:1) and linolenate (C18:3n3) in milk fat, were significantly affected by different levels of dietary Ca and P (P<0.05). Along with the increase of dietary Ca and P levels, the content of saturated fatty acid (SFA) in milk fat was increased while the content of unsaturated fatty acid (USFA) was decreased. Growth development indices (BW, body length, body height and circumference of cannon) and the contents of plasma Ca, P, parathyroid hormone (PTH) and bone Gla protein (BGP) of foals showed no significant differences when mares were fed different levels of dietary Ca and P diets (P>0.05). With the increase of dietary Ca and P levels, chest girth of foals declined by a certain extent. Moreover, plasma calcitonin content of foals in the group Ⅰ was significantly higher than that of groups Ⅱ, Ⅲ and Ⅳ with an increase of 34.24%, 24.89% and 23.64% (P<0.05), respectively. In conclusion, increasing dietary Ca and P levels can improve the milk yield of Yili mares, but reduce the milk Ca content. Dietary Ca and P levels do not affect the milk composition, but increase saturated fatty acids and reduce unsaturated fatty acids contents in milk fat. Lower milk Ca content may decrease plasma calcitonin content of foals, and hence slow the chest girth development of foals.