The CP-violating weak phase ϕs of the Bs0 meson and the decay width difference ΔΓs of the Bs0 light and heavy mass eigenstates are measured with the CMS detector at the LHC using a data sample of Bs0→J/ψϕ(1020)→μ+μ−K+K− decays. The analysed data set corresponds to an integrated luminosity of 19.7fb−1 collected in pp collisions at a centre-of-mass energy of 8TeV . A total of 49 200 reconstructed Bs0 decays are used to extract the values of ϕs and ΔΓs by performing a time-dependent and flavour-tagged angular analysis of the μ+μ−K+K− final state. The weak phase is measured to be ϕs=−0.075±0.097(stat)±0.031(syst) rad , and the decay width difference is ΔΓs=0.095±0.013(stat)±0.007(syst) ps−1 .