As you haven't said whether you're asking about winter or summer flowering Jasmine, I'm including the RHS advice on both - I hope it helps.
"Summer jasmine: Is best pruned just after flowering, in late summer or early autumn. Early flower flushes develop on the previous year’s growth, but later flushes develop on the tips of the current year’s growth. Pruning after flowering gives the new growth time to mature and flower early next season.
Winter jasmine: Is best pruned in spring, immediately after flowering. Flowers develop on the previous year’s growth. Pruning after flowering gives the new growth time to mature and flower next season."
28 Sep, 2014
As you haven't said whether you're asking about winter or summer flowering Jasmine, I'm including the RHS advice on both - I hope it helps.
"Summer jasmine: Is best pruned just after flowering, in late summer or early autumn. Early flower flushes develop on the previous year’s growth, but later flushes develop on the tips of the current year’s growth. Pruning after flowering gives the new growth time to mature and flower early next season.
Winter jasmine: Is best pruned in spring, immediately after flowering. Flowers develop on the previous year’s growth. Pruning after flowering gives the new growth time to mature and flower next season."
29 Sep, 2014