This study is a prospective, single-center, parallel-design,1:1 randomized controlled trial with triple blinding. It aims to investigate the effects of fecal microbiota transplantation (FMT) on cognitive function in patients with long-term type 1 diabetes (T1D), as well as its effects on other complications, glycemic control, insulin dosage, insulin resistance, peripheral blood immune cells, serum metabolites, and safety. This study is divided into two phases for recruiting participants. The first phase recruits 10 individuals (experimental group: 5, control group: 5), while the second phase recruits 30 individuals (experimental group: 15, control group: 15).The main research objectives are as follows: 1. To observe the difference in cognitive function between the FMT group and the placebo group, with the indicators including cognitive scale scores, changes in brain MRI imaging indicators and brain age at week 24 compared to baseline. 2. To observe the difference in other complications between the FMT group and the placebo group, with the indicators including changes in urinary albumin/creatinine ratio, fundus photography, carotid intima-media thickness, and arterial pulse wave velocity at week 24 compared to baseline. 3. To observe the difference in glycemic control, insulin dosage, insulin resistance, peripheral blood immune cells, gut microbiota, and serum metabolites between the FMT group and the placebo group. 4. To evaluate the safety of FMT.