A　reverse　Brayton　cycle　with　advance　refrigeration　of　liquid　nitrogen　and　four　cycles　with　advance　expander　refrigeration　were　designed　to　provide　17.5　K　cold　source　for　H2　system　of　cold　neutron　source　(CNS)　project.　Based　on　thermodynamic　and　exergetic　computation　for　the　different　cycle　modes,　thermodynamic　parameters,　exergy　loss,　exergy　efficiency　and　cycle　capability　were　obtained　and　analysed　respectively.　The　results　show　that　the　capabilities　and　exergy　efficiencies　of　the　cycles　with　liquid　nitrogen　and　prepositive　parallel　connected　expander　are　greater　than　those　of　others;　for　the　cycle　modes　with　advance　expander　refrigeration,　parallel　connection　mode　is　superior　to　series　connection　mode,　and　prepositive　mode　is　superior　to　postpositive　mode.　The　main　exergy　loss　of　cycle　occurs　in　the　compressor,　expander　and　exchanger,　so　there　are　three　ways　to　improve　exergy　efficiency　of　cycle:　(1)　reducing　the　mass　rate　of　helium　by　improving　cycle;　(2)　improving　isoenthopic　efficiency　of　the　expander　and　isothermal　efficiency　of　the　compressor;　(3)　improving　distribution　of　temperature,　pressure,　temperature　difference　and　refrigerant　in　the　exchanger.　These　results　provide　numerical　basis　of　the　design　of　helium　cryogenic　cooling　system　of　CNS　for　China　advanced　research　reactor　(CARR).