In　order　to　investigate　the　swarm　parameters　of　SF6　alternative　gases,　a　high-precision　pulsed　Townsend　experimental　setup　was　built.　With　the　principle　of　pulsed　Townsend　measurements,　the　swarm　parameters,　including　the　effective　ionization　rate　coefficient,　electron　drift　velocity　and　density-normalized　longitudinal　diffusion　coefficient,　could　be　obtained.　The　experimental　setup　and　measurement　methods　were　described　in　detail,　as　well　as　the　swarm　model　for　signal　analysis.　The　setup　and　methods　were　verified　with　measurement　in　CO2　at　10kPa,　with　the　(E/N)　range　from　0　to　130Td.　The　present　results　were　compared　with　experimental　reference　data,　and　the　swarm　parameters　calculated　by　a　Boltzmann　equation　solver.　The　effective　ionization　rate　coefficient,　electron　drift　velocity　and　density-normalized　longitudinal　diffusion　coefficient　in　CO2　were　successfully　measured,　and　the　critical　electric　field　strength　(E/N)cr　for　CO2　was　obtained　to　be　(86±1)　Td　according　to　the　measured　effective　ionization　rate　coefficient.　The　good　agreement　was　found　for　the　effective　ionization　rate　coefficient　between　the　present　data　with　references.　Meanwhile,　good　agreement　could　be　found　when　comparing　the　experimental　results　with　reference　data,　and　the　possible　error　were　analyzed.　Furthermore,　some　tests　on　pure　C4F7N　were　done,　obtaining　its　effective　ionization　rate　coefficient　at　100　Pa.　The　experimental　apparatus　and　methods　will　be　used　to　measure　swarm　parameters　and　evaluate　the　dielectric　strength　of　SF6　alternative　gases,　providing　significant　fundamental　data　for　further　study　on　their　discharge　mechanism.　©　2021　Chin.　Soc.　for　Elec.　Eng.