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<p>Previous studies have shown that <i>BMPR1B</i> promotes follicular development and ovarian granulosa cell proliferation, thereby affecting ovulation in mammals. In this study, the expression and polymorphism of the <i>BMPR1B</i> gene associated with litter size in small-tail Han (STH) sheep were determined. The expression of <i>BMPR1B</i> was detected in 14 tissues of STH sheep during the follicular phase as well as in the hypothalamic–pituitary–gonadal (HPG) axis of monotocous and polytocous STH sheep during the follicular and luteal phases using quantitative polymerase chain reaction (qPCR). Sequenom MassARRAY^® single nucleotide polymorphism (SNP) technology was also used to detect the polymorphism of SNPs in seven sheep breeds. Here, <i>BMPR1B</i> was highly expressed in hypothalamus, ovary, uterus, and oviduct tissue during the follicular phase, and <i>BMPR1B</i> was expressed significantly more in the hypothalamus of polytocous ewes than in monotocous ewes during both the follicular and luteal phases (<span class="inline-formula"><i>P</i>&lt;0.05</span>). For genotyping, we found that genotype and allele frequencies of three loci of the <i>BMPR1B</i> gene were extremely significantly different (<span class="inline-formula"><i>P</i>&lt;0.01</span>) between the monotocous and polytocous groups. Association analysis results showed that the g.29380965A<span class="inline-formula">&gt;</span>G locus had significant negative effects on the litter size of STH sheep, and the combination of g.29380965A<span class="inline-formula">&gt;</span>G and <i>FecB</i> (Fec – fecundity and B – Booroola; A746G) at the <i>BMPR1B</i> gene showed that the litter size of AG–GG, AA–GG, and GG–GG genotypes was significantly higher compared with other genotypes (<span class="inline-formula"><i>P</i>&lt;0.05</span>). This is the first study to find a new molecular marker affecting litter size and to systematically analyze the expression of <i>BMPR1B</i> in different fecundity and physiological periods of STH sheep.</p>