The　experiments　on　two-phase　flow　instabilities　in　parallel　vertical　internally　ribbed　tubes　were　conducted　to　investigate　the　hydrodynamic　flow　characteristics　under　low　mass　flux　conditions　in　a　600　MW　super-critical　W-shaped　flame　boiler.　Two　kinds　of　oscillations,　pressure　drop　oscillation　(PDO)　and　density　wave　oscillation　(DWO),　were　observed.　The　influences　of　pressure,　mass　flux,　inlet　sub-cooling,　compressible　volume,　exit　throttling,　and　asymmetric　heat　flux　on　the　two-phase　instability　were　explored　and　analyzed.　The　results　show　that　the　PDO　occurred　at　lower　exit　quality　and　in　the　negative　slope　portion　of　the　Δp-G　curves　with　the　in-phase　oscillating　of　both　channel　flow　rates,　and　DWO　occurred　at　higher　exit　quality　and　in　the　positive　slope　portion　of　the　Δp-G　curves　with　the　out-phase　oscillating　of　channel　flow　rates.　Based　on　the　testing　results　and　the　homogeneous　model,　the　threshold　expressions　of　instability　were　obtained.　It　is　suggested　that　these　findings　may　be　used　in　the　water　wall　design　and　its　safe　operation　in　super-critical　boilers.