The endometrium is essential for embryo implantation. The clinical pregnancy rate and live birth rate of patients with thin endometrium are significantly lower than those of normal endometrium. Previous studies have shown that tamoxifen has advantages for improving endometrial thickness. However, there is still a lack of evidence from randomized clinical trials comparing the efficacy between hormone replacement and Tamoxifen combined Of hormone replacement.This is a prospective, randomized placebo-controlled, double-blind clinical trial that includes 120 patients younger than 38 years old with a thin endometrium preparing for frozen embryo transfer. Participants will be randomly assigned (1: 1) into two parallel groups: estrogen replacement and tamoxifen combined with estrogen replacement.Frozen embryo resuscitation transfer cycle for thin endometrium patients。This is the first randomized controlled trial to comparing estrogen and estrogen combined with tamoxifen for endometrial improvement，The results of this study will provide evidence for the efficacy of the strategy of frozen embryo transfer cycle for thin endometrium patients.

Inclusion Criteria:

1) Younger than 38 years old, basal serum level of follicle stimulating hormone <10 IU / L; (2) Endometrium is less than 8mm in at least 2 cycles: superovulation cycle / natural cycle / estrogen replacement therapy / ovarian stimulation cycle; (3) at least 1 high quality frozen embryo; (4) There are no comorbidities that clearly affect pregnancy, such as adenomyosis, endometriosis, and intrauterine adhesions;

Exclusion Criteria:

1. abnormal karyotype;
2. Accompanying other diseases of the uterus: uterine muscular wall myomas that affect the uterine cavity shape, more severe adenomyosis, severe endometriosis, congenital uterine malformations, endometrial tuberculosis, etc .;
3. Contraindications to hormone replacement therapy;
4. Participating in other clinical research;
5. History of previous fundus diseases;