Genetic parameters of major economic traits and their relationships of gametophytic blades in Pyropia yezoensis are the foundation of molecular breeding. A double haploid (DH) population of 152 lines was developed from the heterozygous conchocelis of an intraspecific crossing of wild-type strain (Py-WT2, male parent) and red mutation strain (Py-HT, female parent). It was found that all 6 traits (L50, W50, FW50, SGR-L, SGR-W and SGR-FW) were quantitative traits when analyzed by one-sample Kolmogorov-Smirnov test. The means of 4 traits of DH population lines were between their parents, among which W50 was nearer to female parent, and FW50, SGR-L and SGR-FW were nearer to male parent. L50 and SGR-W showed transgressive inheritance, among which L50 was lower than low value parent (female), and SGR-W was higher than high value parent (female). Coefficient of variation of the 6 traits of DH population was between 21.11%–56.68%, which meant that they had intermediate variability. Highly significantly positive correlations were observed between L50 and W50, L50 and FW50, W50 and FW50, and between SGR-L and SGR-W, SGR-L and SGR-FW, SGR-W and SGR-FW. Heritability of L50, W50 and FW50 were 58.17%, 64.00% and 57.64%, respectively; and the number of genes controlling the 3 traits was 6.61, 12.63 and 8.09, respectively. The equation of the curve between heritability (y) and the number of genes (x) was y=0.2922x²-4.6533x+76.162 (R²=1). According to estimated coefficients of skewness and kurtosis of the 3 traits tested, gene interactions were found to be absent for L50 and FW50, respectively, and complementary interactions were observed in W50 and SGR-W respectively, and duplicate interactions were observed in SGR-L and SGR-FW respectively. The results indicated that L50 and FW50 could be selected at early generation by their high heritabilities, less controlling genes and no gene interactions, and indirect selection could be applied between L50, W50 and FW50 to improve breeding efficiency of P. yezoensis because of their high correlations between each other.