This paper reports the changes in properties and structure of theagar from Porphyra haitanensis under various concentration of alkali-pretreatment.Having been treated with concentrated alkali, chemical component and rheologicalproperties of the solutions change remarkably. The content of sulfate ester (SO₃)decrease from 5.8% to 0.12% 3.6-anhydrogalactose increase from 6% to 44. 50%.the gel strength (g/cm²) increase from 0 to 761. It is suggested that the de-esteri-fication of sulfate residues increased the gel forming power. This is caused by the increase of hydrogen bond and of 3.6-anhydro-L-galactose. These two factorsmake agar molecules stablize of double helix structure. The IR spectrum and electron microphotograph show a great difference be-tween agar porphyra and porphyran. 13c-n. m. r. spectra of alkali treated porphyrapolysaccharide further show that the results of alkali treated is agar. This agar aswell as other agar is double helices formed the microcrystalline structure.