A device for detecting the copy number of fetal chromosomes or tumor cell chromosomes, comprising: a detecting module, which is adapted for sequencing DNA in a sample of pregnant women plasma or plasma of tumor patient; a comparison module, which is adapted for comparing a sequencing result of the DNA with a genomic sequence map to determine which chromosome each DNA sequence comes from and the length of each DNA sequence; a calculating module, which is adapted for calculating the ratio of the number of DNA segments from the chromosomes to be detected to the total number of DNA segments in the same sample, and correcting the ratio according to a GC content of the DNA segments from the chromosomes to be detected; and calculating the variation of the corrected ratio of the DNA segments from the chromosomes to be detected in a sample to be detected, and determining the copy number of the chromosomes to be detected according to degree of variation, wherein the calculating module corrects the ratio according to a GC content of the DNA segments from the chromosomes to be detected according to the following function: the GC contents of the DNA segments from each chromosome respectively have linear relationships with the ratios of the DNA segments from each chromosome to the total DNA segments, and the linear relationship can be represented by y=ax+b, where y represents the GC content of the DNA segment from the chromosome to be detected, x represents the ratio of the number of the DNA segments from the chromosome to be detected to the total DNA, a and b are constants, a and b may be different values for different chromosomes; and an output module, which is adapted for outputting the copy number of the chromosomes to be detected. a detecting module, which is adapted for sequencing DNA in a sample of pregnant women plasma or plasma of tumor patient; a comparison module, which is adapted for comparing a sequencing result of the DNA with a genomic sequence map to determine which chromosome each DNA sequence comes from and the length of each DNA sequence; a calculating module, which is adapted for calculating the ratio of the number of DNA segments from the chromosomes to be detected to the total number of DNA segments in the same sample, and correcting the ratio according to a GC content of the DNA segments from the chromosomes to be detected; and calculating the variation of the corrected ratio of the DNA segments from the chromosomes to be detected in a sample to be detected, and determining the copy number of the chromosomes to be detected according to degree of variation, wherein the calculating module corrects the ratio according to a GC content of the DNA segments from the chromosomes to be detected according to the following function: the GC contents of the DNA segments from each chromosome respectively have linear relationships with the ratios of the DNA segments from each chromosome to the total DNA segments, and the linear relationship can be represented by y=ax+b, where y represents the GC content of the DNA segment from the chromosome to be detected, x represents the ratio of the number of the DNA segments from the chromosome to be detected to the total DNA, a and b are constants, a and b may be different values for different chromosomes; and an output module, which is adapted for outputting the copy number of the chromosomes to be detected.