To explore the relationship between magnesium(Mg) nutrition and tomato growth and leaf enzyme activity under solar greenhouse condition，a hydroponic pot experiment was conducted using tomato by five magnesium levels，including 0，0.5，1.0，1.5，3.0 mmol/L，which were represented by Mg0，Mg0.5，Mg1.0，Mg1.5，Mg3.0，respectively.The effects of different magnesium levels on shoot，root，photosynthetic parameters and leaf enzyme activity of tomato at different growth stages were measured.The results could provide theoretical and practical basis for accurately diagnosing the symptoms of magnesium nutrition balance and correcting magnesium deficiency measures under facility condition.The result showed that(1)In the three growth stages of tomato，the plant height and stem diameter of Mg0 decreased by 41.3% and 20.4% compared with Mg1.0，and those of Mg3.0 decreased by 26.2% and 12.4% compared with Mg1.0；In the latter two growth stages，the number of branches of Mg0 and Mg3.0 decreased by 23.3% and 12.1% compared with Mg1.0，and the number of inflorescences decreased by 33.3%.When magnesium was deficient or excessive，the leaf area was also significantly smaller than that of the appropriate treatment.(2)The total length，surface area，volume and diameter of roots under Mg0 and Mg3.0 treatments at seedling stage were significantly less than those under Mg1.0 and Mg1.5，respectively.At flowering and fruit setting stage，the volume，average diameter and projected area of roots under Mg0 and Mg3.0 treatments were significantly greater than Mg1.0，and the roots became significantly shorter and thicker.(3) The change range of SPAD among the three treatments with Mg2+ concentration of 0.5 ~ 1.5 mmol/L was small.Mg0 and Mg3.0 significantly reduced SPAD by 143% and 15.6% respectively compared with the average value of the three treatments.There was no significant difference with SPAD value among the magnesium treatments at fruit maturity.(4) The activities of peroxidase(POD) and catalase(CAT) in seedling stage and flowering and fruit setting stage gradually increased with the increase of magnesium concentration，and the activity of Mg3.0 decreased significantly at fruit ripening stage.With increase of magnesium concentration，superoxide dismutase(SOD) gradually decreased in seedling stage，decreased at first and then increased in flowering and fruit setting stage，and increased continuously in fruit ripening stage.In conclusion，magnesium concentration mainly affects the shoot，root growth and leaf physiological and biochemical characteristics of tomato at seedling stage and flowering and fruit setting stage.At this stage，when the magnesium concentration is 1.0 mmol/L，the enzyme activity is appropriate，which is conducive to the formation of various growth parameters of shoot and root of tomato.The leaf color and leaf area reach the best level.Lack of magnesium or excessive magnesium significantly inhibit the growth and physiological and biochemical characteristics of tomato.